Was Peter Ortiz a real person?

Yes, he as a real person. He was in the French Foreign Legion and re-upped at the beginning of WWII. He was captured by the Germans in France, escaped, and made his way to the US where he signed up with the US Marines. In the Marine Corps, he worked with the OSS, the forerunner of the CIA. Many Marines still work with the CIA (see LT. Col. North).

I don't know that everything you read about him can be verified. His life reads like that of a super hero. He was said to have been perturbed by some Germans in France when he was working with the French underground. Supposedly, they made derogatory remarks about the US, our president, and worst about his beloved Marine Corps. The story is told that he left the bar and came back with pistols and in uniform under a coat. Removing his coat, he held the German officers at bay with his pistols and had them drink toasts to the US and the Marine Corps. Whether that really happened or not, I can't say. It is exactly the kind of thing one might expect from him.

What did happen was he and two other Marines held off a battalion of Germans, moving around and making them think they were facing a large force. When they finally surrendered in exchange for the Germans not harming the villagers, the Germans couldn't believe that they had been facing only 3 Marines. They demanded that all of them surrender. It took a while to convince them that there were only 3 of them!!