who is the enemy in the movie, and why?

The movie (both versions) follows the book pretty well. On the surface, the enemy would be the allied forces fighting against the Germans. Paul, however, comes to realise that he shares a greater kinship with "enemy" combatants that he does with most civilians back home in Germany. Paul finds the enemy to be the politicians and high ranking generals who give the orders to fight such a brutal and pointless war. These men sacrifice the lives of boys and men over issues that have nothing to do with the men that are dying. At one point Paul fantasizes that the world's leaders merely meet in a ring and fight it out themselves.