The opening conversation is almost a battle of words between Colonel Graff and Admiral Chamrajnagar, the head of Command School. They discuss what Ender still needs to be told.
Ender hates Eros from the moment they land. Everything feels odd, and he is surrounded by strangers. He is isolated as he gets individual tutoring from teachers. Because of this isolation, Ender throws himself into his studies, learning quickly and well. His favorite activity is to play on the simulator, essentially "the most perfect videogame he had ever played." Ender starts in control of just one fighter ship, but he is soon allowed to "command" a four-fighter squadron. The computer starts giving him specific objectives. Ender notices that after he uses a tactic against it, the computer uses it back against him in a matter of minutes, learning like an enemy would. Ender learns to spread his focus over the entire group of ships, as a real commander would.
Finally, after a year of Command School, Ender seems to have exhausted the simulator's abilities. Also, he wants something like toon leaders who can think for themselves.
After making this request, the next day there is an old man sitting cross-legged on his floor. Ender waits for him to speak, but he does not speak even when addressed. His door is locked, so he does his morning exercises. One time the man grabs him but then retreats, so Ender ignores him. When Ender's guard is down, the old man grabs him violently and tells him that he should never turn his back on the enemy. This man will be his teacher and his enemy, because "there is no teacher but the enemy." As the man lets him go, Ender kicks him, and they fight. They hurt each other, and when Ender finally asks the old man his name, his new teacher says: "Mazer Rackham."
Mazer is now too old to command (he was kept alive via a relativity maneuver, aging eight years while the Earth passed fifty years), but he can teach. They watch videos from the First and Second Invasions and discuss them. Mazer points out tactics and strategies that Ender never noticed before, and "for the first time, Ender had found a living mind he could admire."
Ender is curious about how Mazer won against the buggers by targeting just one ship. Mazer is not sure, but he thinks that the buggers' instantaneous mind-speak is not really a conversation at all. It seems that one bugger, the Queen, controls all of the others. When Mazer destroyed the Queen ship, all of the other buggers were destroyed along with her. "Every ship acts like part of a single organism."
Mazer also guesses that the First Invasion was exploratory, but the Second was a colonizing trip, and this is why they brought a queen. They closely examine the video, and Ender eventually can discern which ship is the Queen's. They are the only two people who ever could.
Mazer also notes that since Eros was originally a bugger colony, which is why Ender feels so uncomfortable in his surroundings there. Much of the technology that the Fleet uses now comes from things that they found on Eros, such as gravity manipulation and the use of stellar energy to black out planets. Mazer also guesses that the buggers killed the first humans they saw without realizing that they were killing intelligent life, thinking that the outlying humans were worthless because none was a "queen." In this respect, human ships have an advantage in that each one is commanded by intelligent life.
Moreover, the humans' weapons have an enormous range, except for old versions of their newest weapon, called Little Doctor (or Dr. Device, which came about because of its scientific name, "Molecular Detachment Device," or M.D. Device). The Little Doctor splits apart molecules and causes a chain reaction in a spherical area, though it gets weaker as the radius increases and picks up strength when it hits new molecules. The M.D. Device can penetrate the buggers' shields.
In the simulator room, Ender starts training as a commander of squadron leaders, much like with his old toon leaders. He puts on the headphones and hears "Salaam," indicating Alai's presence. Bean, Petra, Dink, Fly Molo, Crazy Tom, Hot Soup, Carn Carby, Shen, and "all the best students that Ender had fought with or fought against, everyone that Ender had trusted in Battle School" are all there to work with him. They practiced together for three weeks before the "battles" began, and Ender grew to know and understand them, their strengths and weaknesses, perfectly. They learned to trust each other completely. They had the almost instantaneous reactions of a bugger fleet, but they also clearly had the advantage because his subordinates could still make their own decisions. Mazer says that from now on, he will be controlling the simulation of the enemy ships--he warns Ender that he will grind him down to the breaking point to make sure that Ender will not fail later.
In the first battle, the enemy outnumbers Ender's group two to one, but they are grouped together tightly, so the Dr. Device easily wins. The enemy quickly learns not to group the ships together again, while Ender learns from his own mistakes. Over several more battles, the squadron leaders' trust in Ender as their commander grows, though their friendship with him gradually disappears, and they grow closer to each other while growing apart from him. "Ender was their teacher and commander, as distant from them as Mazer was from him, and as demanding."
After more battles, usually twice a day, Ender starts having nightmares about the fantasy game. Mazer wakes him one day for a battle while Ender is calling out in his sleep, to tell him that they have another battle. Ender is sleeping less and less. He and his squadron leaders begin to make more and more mistakes, and then one day, Petra becomes numb at the controls. She loses all but two of her fighters by the time Crazy Tom takes over, and they almost lose the battle. Petra is broken, no longer as reliable, and Ender relies on her less. The stress on Ender increases as well, and his nightmares get worse. One night he wakes up with a bloody hand from gnawing on it during his sleep. He tells Mazer that he thinks he is going insane
One day during practice, everything goes black, and Ender wakes up next to the simulator with a bloody face where it hit the controls. He is very sick for three days, and when he eventually wakes up, he has an immediate battle. During the night after that battle, he "dreams" he heard voices saying that he was almost finished, and that both voices (they sound like Graff and Mazer) said that they cared about him. Ender grew into a cycle for the next several days by which he woke up, won a battle, went straight back to sleep, and then repeated the pattern.
Finally, Ender's last day of Command School comes, though he does not know it. In the simulator room there is a large audience. Mazer says that it is Ender's final test in Command School. This battle will be situated around a planet, unlike all his other battles, and when Ender asks if the Little Doctor would work against a planet, Mazer replies, "Ender, the buggers never deliberately attacked a civilian population in either invasion. You decide whether it would be wise to adopt a strategy that would invite reprisals."
When the screen shows the battle area, Ender realizes that the enemy outnumbers his ships "a thousand to one." They are situated in a shifting pattern around the planet, with no obvious lanes for Ender's 80 fighters to go through. Ender realizes that this battle is hopeless. He believes that after all that he has been through, the teachers want him to fail, maybe expect him to fail.
Bean refers to the hopelessness of the battle by saying, "Remember, the enemy's gate is down," an allusion to Dragon Army's seemingly hopeless battle against the combined Griffin and Tiger Armies. This comment may be a signal to Ender to again bend or break the rules in some way or to use a similar strategy as before, and it at least inspires Ender's strategy. He quickly assigns his squadron leaders to take control of their few ships and shape them into a cylinder, pointing toward the planet. As the enemy formation closes in on them, Ender's fleet melts "into chaos," with his ships firing at any and all of the enemy ships and getting cut up themselves. After a few minutes of this feint, Ender organizes twelve remaining ships into a formation about halfway to the planet. At Ender's command, the ships drop straight toward the enemy's planet to use the M.D. Devices. At least one Little Doctor reaches the planet with its effects, because the planet suddenly begins to bubble, and within three seconds "the entire planet burst apart, becoming a sphere of bright dust, hurtling outward." Only the starships on the outer edge of the battle survive, including just two or three of the enemy ships. Gravity starts pulling the debris from the explosion back together.
As Ender removes his headphones, the room is filled with noise of people laughing, shouting, weeping, and so on. Graff thanks him, and Ender is confused, since "it was his victory, not theirs, and a hollow one at that, a cheat; why did they act as if he had won with honor?" Mazer tells Ender that he made a hard choice, "End them or end us...Congratulations. You beat them, and it's all over." Mazer reveals that the battles were real, with buggers, the whole time. Ender battled the buggers at their home world, where all of their queens were, and he destroyed them all.
While others see Ender as a hero, he worries that "I killed them all ... All their queens. So I killed all their children, all of everything." Ender weeps again, saying that he never wanted to kill anyone and that he has been tricked into being Peter. Graff responds that only Ender could win the love of his underlings, understand and anticipate the buggers, yet go into a battle "willing to win at all costs."
Back on Earth, the League War is initiated by the Warsaw Pact. It lasts a short while before there is a truce according to the "Locke Proposal." The war reaches Eros, but the Russians love Ender and he remains safe. Russia, meanwhile, has its own problems with a rebellion of the Islamic States.
Ender talks with some of his leaders. When Alai asks what they will do when they get back to Earth, Petra points out that they are kids, so they might have to go to school, and the friends all laugh.
When Mazer tells Ender, "There is no teacher but the enemy," the idea is a variation on Ender's perception that the teachers are the enemy. Ender also has let the buggers teach him, somewhat, having studied videos of the bugger invasions for hours a day in Battle School. Throughout the novel, Ender's way of defeating his enemy is to understand him. Ender saved himself in fighting Bonzo, for instance, by appealing to his honor. If Mazer understands that Ender already knows that the enemy is his teacher, he must be attacking Ender when they meet for another reason. For one thing, the lesson is never to turn his back on his enemy--because if he does, even for a second, his opponent will take advantage of it and destroy him. This actually happens during a battle, when Petra loses almost all her ships after becoming numb at the controls. For another thing, Mazer must prove that he is worthy of being Ender's teacher, partly through a macho fight (though it is an old man against a young boy), and partly through superior patience and speed.
Ender and Mazer have one very important thing in common: they both try to understand the enemy, not just defeat him. When they discuss Mazer's battle during the Second Invasion, they try to figure out why the buggers did not stop him from shooting the queen's ship. Ender points out that they "could have blown [Mazer] out of the sky," but did not. Mazer comments that he thinks that it may not have occurred to any of the buggers that he would kill their queen. "Maybe in their world, queens are never killed, only captured, only checkmated. I did something they didn't think an enemy would ever do." Mazer also has had many years to contemplate what the buggers might have thought about their intentions and about the humans. Their attempt at understanding the buggers is what makes Ender and Mazer great commanders.
Since they are the only characters who attempt to understand the buggers, they begin to feel sorry for them. Ender's compassion is his most redeeming characteristic for all of the pain he has caused (to Stilson, Bonzo, and then the buggers). Ender's compassion for the buggers makes him able to destroy them, but later it will enable him to give them the chance to start over by saving their last queen. Compassion is extremely important throughout the novel--Valentine's compassion for Ender encourages him when he is ready to give up, for example.
When Ender first arrives on Eros, he feels uncomfortable, and eventually Mazer tells him that Eros was originally a bugger colony. Humans killed them all, and Mazer notes, "This was the treasure trove." The buggers left technology and information on Eros, including gravity manipulation machines and machines that used stellar energy to black out the planet, making it darker than a black hole. Therefore, the humans had the buggers to thank for much of their advanced technology, including the ansible and the ability to manipulate rooms such as the battleroom at Battle School. In other circumstances, the humans and buggers could have traded technology peacefully.
Indeed, there are many times during Ender's training that he questions whether they should be fighting the buggers at all. He has many such conversations with Graff on the way to Command School, but he brings it up just once with Mazer. Mazer tells him that when the I.F. sent a ship to figure out why Eros had suddenly been blacked out, the buggers had boarded their tug and systematically killed all of the crew, and only after that, the buggers began to examine the entire ship and dismantle their technology, including their transmission machinery. Ender asks Mazer why the buggers killed that first crew, and Mazer hypothesizes, "To them, losing a few crew members would be like clipping your nails. Nothing to get upset about. They probably thought they were routinely shutting down our communications by turning off the workers running the tug. Not murdering living, sentient beings with an independent genetic future." That is, in the bugger understanding of life, only the queens really matter. Ender's reaction to this is to point out that if they were truly innocent, they should not be punished by a third invasion. Mazer, however, replies that killing life, especially human life, is an evil in itself. They were acting like Peter, who would kill a squirrel without remorse but who only draws the line (apparently) at killing other people. Yet, if the humans were not really considered life but merely part of a communication system (which would be hard to believe), then the bugger intentions would be excusable from a perspective of relativism, still not excusable from a persepctive of the inherent value of human life.
The issue here is that humans can all think individually, and the buggers have no such experience of that. Mazer tells Ender that one of their major advantages against the buggers is that "every single one of our ships contains an intelligent human being who's thinking on his own," unlike the buggers, who are all controlled by one central queen. This advantage finally brings into play one of Ender's important strategies when he became a Commander in Battle School. He kept his toons completely fluid, including giving his toon leaders much more power than they would have had under any other commander, because he realized that it gave them an advantage to be able to make up their own decisions instead of having to continuously report back to him for orders. Ender's form of commanding, therefore, is almost completely opposite to the buggers'. He has the strong advantage of being able to trust his subordinates to make good decisions when he might not be around. More generally, this preference of individualism versus central planning reflects the American versus Soviet style of governance during the author's experience of the Cold War. (This conflict remains present down to Ender's day, with the League War between Russia and America.)
Even though the squadron leaders believe they are playing simulations, the stress on them, and on Ender, is extreme. Petra is the first of Ender's squadron leaders to "break," and Ender becomes worried because he knows that she has been far from the weakest of his subordinates. Petra's fall teaches him to be wary of the stress on himself, and it teaches him to consider the stamina of his leaders as well as their other characteristics. Ender is no superhero; he says, "Part of what I am is her ... what she made me." Ender knows to beware of his own limitations; his teachers have not been perfect, not even the computer that ran the fantasy game.
Ender is never told that he killed Bonzo during the bathroom fight, and though he suspects it from the beginning, he never seems to fully understand that he did so until his dreams at Command School. While he was still in Battle School, Ender told Dink that it seemed like Bonzo was dead standing up, though it seemed that Bonzo was merely unconscious. During his dreams at Command School, however, as Ender replays his memory of the fight over and over again, he decides that Bonzo must have been killed when Ender head-butted his nose up into his brain. Mazer never refutes this idea, but he does try to calm Ender down by saying, "It was just a dream." Ender's life is pretty much a never-ending cycle of killing people and not knowing it at this point-nobody tells him he is killing the buggers.
Ender's "dreams" during Command School seem to be an outlet for truth in his life. In his dreams, Ender realizes that he killed Bonzo. Also, he dreams of the buggers "vivisecting" his memories, and we later find out that they were studying him and his memories in order to rebuild part of the Fantasy Game in real life. Finally, he "dreams" about conversations between Graff and Mazer while he is half-awake, but he does not believe that they are true, because in the conversation, they seem unusually affectionate toward him. One of them says, "I can't bear to see what this is doing to him," and the other replies, "I know. I love him, too." Graff is still his friend, though Ender has not known it. And Mazer has been his friend all along, despite acting as Ender's enemy.
In his last battle at Command School, Ender has completely given up. He does not even want to win, because he thinks that if he wins, he will have to go through another several years of training, and then will have to command the International Fleet's ships against the buggers, which might take several more years. And once again, the test seems to be against impossible odds. Therefore, Ender enters the battle wanting to lose, because he thinks, almost subconsciously, that if he were to lose this battle, they would send him home, to Greensboro, North Carolina, and he could do nothing substantial for a very long time. When Ender sees the simulation and the hopelessness of the situation, he decides that they want him to lose. Since Mazer is going to beat him unfairly, Ender might as well beat him unfairly first.
When Bean says, "Remember, the enemy's gate is down," it gives Ender his strategy for the battle. Interestingly, Ender does not expect to win with the strategy-rather, it is another one of Ender's attempts to "go out in style," like he did during his last battle against the combined Griffin and Tiger Armies at Battle School. Ender thinks to himself, before he commands his squad leaders, "Forget it, Mazer. I don't care if I pass your test, I don't care if I follow your rules. If you can cheat, so can I ... If I break this rule, they'll never let me be a commander. It would be too dangerous. I'll never have to play a game again. And that is victory." This is the reason that Ender decides to use the Little Doctor against the planet, not part of a grand, last-ditch effort at victory, as his audience believes. It is not even a rational decision that it is the only way to destroy the buggers, which certainly is the case. His attack against the planet itself is a last-ditch effort to free himself from their game, and to do so with style. Ender is not even sure the computer would know how to handle the effects of the Little Doctor on a planet.
Ender's final game, in terms of war, is his last battle against the buggers. He looks at it like a game, and he cheats in it, and the cheating is what allows him to win the war. Even though there could be major risks of repercussions for killing off the bugger civilian population, that is not really part of the simulation, so it is not something Ender needs to consider very carefully. The horizon of each training battles is hours, not even days (although the computer supposedly learns his strategies from one battle to the next).
By the end of the novel, the line between games and reality has become completely blurred--the games have been reality after all. Games often seem like reality, and reality seems like a game--and the games can express or even be reality. If reality has the character of a game, its rules can be stretched and broken, just as advancing technology enables people to do what they never "could" before. Of course, breaking the rules has consequences, and if someone does whatever is necessary in order to win, there is a risk of alienating oneself from those who follow the rules. If, for example, another alien civilization sees what the humans did to the buggers, breaking the universal moral imperative of not killing innocents, the humans may be in for trouble in the intergalactic community.
But Ender has never thought of himself as the kind of rule-breaker who kills unnecessarily. He insists, to himself and others, that he is not a killer, despite what they might think, and, in fact, despite what he himself sometimes worries. Usually, these outbreaks include him insisting that he is not Peter. The most notable of these outbreaks are after he kills Stilson, when he insists to himself that he is not like his brother, then when he kills Bonzo and tells both himself and Dink that he is not Peter, and when he kills all of the wolf-children in the fantasy game. His final instance of this self-reassurance is after his last battle against the buggers, when he is told that it was all real. "Ender grabbed Mazer's uniform and hung onto it, pulling him down so they were face to face. 'I didn't want to kill them all. I didn't want to kill anybody! I'm not a killer! You didn't want me, you bastards, you wanted Peter, but you made me do it, you tricked me into it!'" Ender keeps getting put into situations where he is forced to hurt others, the thing he hates worst in the world, and this latest situation is even worse, because he did not know it, and because he killed all of the buggers. When Ender breaks the rules, he sometimes feels alienated from himself.
After Ender's outburst, Graff tells him, "Of course we tricked you into it." He explains that they needed a commander with enough empathy that he would be able to understand the buggers and anticipate their actions during the battles. The bind was that their commander had to have enough compassion to "win the love of his underlings and work with them like a perfect machine, as perfect as the buggers," but at the same time, had to have the killer instinct to go into battle "willing to win at all costs." Such a commander would be impossible to find, because someone with so much compassion could not have that intense killer instinct. "If you were the kind of person who would do it even if you knew, you could never have understood the buggers well enough." Therefore, Graff and Mazer completely tricked Ender into killing all of the buggers, but for the good of humanity; they knew that they had to have a person with this exact combination of characteristics to serve as their commander.
After the bugger war and the League War end and Ender finally wakes up, Petra, Bean, Dink, and Alai join Ender in his room. Whether or not they are his friends, they respect him greatly, and here they do seem to be his friends. The closing joke about returning to school demonstrates the folly of laws that cannot really apply to everyone fairly, and it reminds readers that the children really all are very young. It makes sense why Ender might be chosen to be the supreme commander, but why would all his squadron leaders be children as well? It seems to be because of their pre-existing relationships and the unusual and unprecedented battle strategies that they all have learned together, mostly under Ender's initiative. In any case, it is amazing to think that these children still have many more decades to live.
Throughout the novel, Ender repeatedly has to sacrifice himself for others. In this sense he is like Jesus Christ, although a better comparison might be to Achilles, who sacrifices himself for glory and over against a leader he despises (Agamemnon, in the Iliad), giving up a quiet life at home for the sake of a soldier's life of struggle. When Graff, Mazer, and the other I.F. teachers and officers have finished with Ender, he is barely human. They have destroyed the child in him, though he always was too mature to act like a normal child, but they also have destroyed his adulthood, making him the kind of warrior who has virtually nothing to return to on Earth and whose characteristics have been formed for times of war rather than times of peace. Will Ender Wiggin be able to survive and enjoy himself in times of peace? He has become something of a savior and martyr, but he has the potential for a long life ahead of him. What next, after saving humanity?