The Man in the High Castle is set in a parallel world in which Japan and Germany won World War II. Because The Man in the High Castle tells the story of an alternate history, a solid understanding of actual history is essential for understanding the novel.
World War II was a massive war across Europe, Asia, and North Africa that involved hundreds of millions of people and left nearly 73 million dead. The war was between the Axis powers (Germany, Japan, and Italy) and the Allies (Britain, the Soviet Union, and the United States. France and China are sometimes considered Allies, but they were invaded and taken out of combat early in the course of the war). Though the seeds of the conflict were sewn much earlier, historians generally consider the invasion of Poland by Germany in September 1939 to be the beginning of World War II. Britain and France declared war when Germany violated the boundaries of Poland. The war in Asia began in 1937 with the Marco Polo Bridge Incident, when Japan invaded an annexed China.
In 1940, Germany invaded and annexed the Netherlands, Belgium, Luxembourg, and France. Italy, under Benito Mussolini, invaded southern France, and this major power fell to the Axis powers. Italy also took over Ethiopia.
In 1941, Germany attempted Operation Barbarossa, a massive invasion of the Soviet Union. Initially the invasion was a great success, but the Russians halted the Germans at Stalingrad and began regaining territory, resulting in an eventual invasion of Germany. (Interestingly, a similar turn of events occurs in The Grasshopper Lies Heavy.)
The United States did not actually enter the war until 1941, when the Japanese attacked a military base at Pearl Harbor. Franklin Roosevelt had long wanted to get involved in the conflict, and was able to leverage this vicious attack to overcome the isolationist sentiment of other Americans. The attack at Pearl Harbor damaged a number of ships, but did not wipe out the entire fleet because no aircraft carriers were in the harbor when the attack occurred.
On June 6, 1944, the Western Allies invaded France’s Normandy coast in a maneuver called “Operation D-Day.” By opening a second front, the Allies were able to draw off German resources from the Russian front, effectively splitting up German military resources. D-Day marked the advance of the Allied powers into German territory.
Germany surrendered unconditionally on May 8th, 1945. Following the atomic bombings of Hiroshima and Nagasaki, Japan surrendered on August 15, 1945.