Paul Bäumer and his friends—Leer, Müller, and Kropp—are old schoolmates. They're all the same age—nineteen—and were all in the same class together at school. Paul and his friends enthusiastically volunteer to enlist, believing, like many millions of other young men across Europe, that the War will soon be over, worried that they might miss out on all the action. However, not all of Paul's classmates were quite so enthusiastic at the prospect of signing up. Joseph Behm didn't really want to go off to war, but he finally did so under the domineering influence of Herr Kantorek, the fanatically nationalist schoolmaster.
It was quite common on both sides during the First World War for friends to serve together in the same regiment. Sadly, these so-called "pals' regiments" tended to have higher mortality rates, which wrought appalling suffering on the communities they left behind.