In chapt. 3, What forces him to fight and prevent him from running?

Henry doesn't actually fight in Ch. 3. Henry doesn't really witness the fighting until Ch. 4. In Ch. 4 Henry witnesses the regiment in front of him run. Despite threats from their superiors they run for their lives transfixed by nothing more than to flee certain death. Henry resolves not to run and to look the "monster" in the face. He does not want to be seen as a coward. In the end Henry does run; this turns out to be standard practice during a first battle.

Oh, Henry also realizes that even if he wanted to run, the throng of surrounding soldiers would trample him.

Thank you so much!(: