Describe some of the early details of the anti-war theme?

This depends how early in the novel you are referring to. We see the boyish excitement about war and the naiveté of the young soldiers marching to it. We certainly get the sense these boys are clueless as to what they are about to experience. When they have their first battle, there is no heroism or bravado. There is only panic and terror among the new recruits as they retreat from the monster that is war.