Who epitomizes nationalism in the novel?

That would be their teacher Kantorek. Although Kantorek, the former schoolteacher of Paul and his friends, figures in only one present-tense scene, he casts a long shadow over the novel. He represents nationalism, the ideology of unswerving dedication to one's own country that swept Europe before and during WWI, at its worst. His patriotic sentiments and bullying forced Paul and his classmates--what he proudly calls the "'Iron Youth'"--into volunteering for the war. Paul gains some measure of revenge when he sees that Kantorek has been enlisted in the war; at least Kantorek must now fight and possibly die for the war he has helped promote.