Think about the concepts of enemies in war. Whom do paul and his friend regard as their enemies?

They see the generals and politicians on both sides who send them to fight as their enemies. They merely kill other soldiers to survive.

Here is a great quote from Chapter 3,

"Kropp on the other hand is a thinker. He proposes that a declaration of war should be a kind of popular festival with entrance-tickets and bands, like a bull fight. Then in the arena the ministers and generals of the two countries, dressed in bathing-drawers and armed with clubs, can have it out among themselves. Whoever survives, his country wins. That would be much simpler and more just than this arrangement, where the wrong people do the fighting."