> It was hard for me at the time to understand why he would regret fighting the Nazis.

In mid-April 1945, the war in Europe was essentially over. Everyone knew it was ending, but Howard Zinn was still ordered to fire bomb the Germans stationed near Royan, France. It was the first use of napalm in warfare actually. His actions wound up burning most of the French men, women, and children in the town to death. This badly traumatized him and likely explains his reluctance to encourage people to fight for the military.