Simon, enlisted in the US Coast Guard, while Kirby enlisted in the US Army and served with Company F of the 11th Infantry regiment. “Kirby landed on Omaha Beach in Normandy on August 23, 1944, two-and-a-half months after D-Day. Kirby recalled that a lieutenant, learning that comics artist Kirby was in his command, made him a scout who would advance into towns and draw reconnaissance maps and pictures, an extremely dangerous duty,” according to his bio on Wikipedia.
One article from TwoMorrows Publishing interviewing Kirby about the war, I thought this portion was notable- “I was a Scout in the infantry. If somebody wants to kill you, they make you a Scout. So, I was a Scout. I don't know who wanted to kill me—maybe somebody that I upset somewhere, I don't know. You don't pull that kind of duty just because you're a nice guy. Nice guys don't get Scout duty. Maybe I was the new guy, so they said, "Give Scout duty to the new guy." That's probably what happened. You don't pick some guy that you like to be a Scout; you'll never hear the end of it…
I remember that I walked into this town where they had the Command Center. This Lieutenant called me over and said, "Private Kirby?" I said, "Yes, sir." He said, "Jack Kirby? The artist?" I said, "Yes sir. I drew Captain America..."
"And Boy Commandos," he said. Lots of guys knew who I was, so this did not surprise me—you have to remember that the Simon/Kirby name was very popular at the time and many adults were reading comic books back then. "So you can draw?" he said. "Yes sir," I said, "Of course I can draw." I was thinking, "Great, some officer wants me to draw his portrait." Then he said, "Good. I am making you a Scout. You go into these towns that we don't have and see if there is anybody there. Draw maps and pictures of what you see and come back and tell us if you find anything."
It’s no wonder Jack Kirby was responsible for the creation of so many heroes...he was a hero himself. You can tell from various interviews that both artists were deeply affected by the war, and you can imagine how differently they viewed the world pre to post war.