This is an enormous question, but I'll try to give you some pointers toward some of the main ideas.
Paul and his friends truly believed Kantorek when the teacher first gave them the line about being the "Iron Youth," which is why so many of them voluntarily signed up to fight, and which is why they all pressured Behm to go along with them to the recruitment office. They buy all the propaganda about Germany just protecting itself from the evil world. They truly believed they were serving the Fatherland.
However, by the time we meet these men in the field after serving at the front, they are disillusioned and bitter. For Paul, the war has become something so large and overpowering, that it takes on an almost mystical quality, calling it "a mysterious whirlpool" that is sucking at him. He invokes the earth with "thy folds, and hollows, and holes, into which a man may fling himself," as though the war is too large for him to confront alone.
Albert Kropp comments that the "war has ruined us for everything," and has a very fatalistic view of it. This is pretty close to Kat's view because Kat clearly sees the war as a political mess that would disappear if officers had to suffer like the enlisted me do. However, it's Kropp who says that war "should be a kind of popular festival" where the leaders fight it out one-on-one.
And with that, I'm out of room to discuss the topic. I hope I gave you a hand with getting started.