In Chapter 3 there is a conversation between Jordan Baker and her friend Lucille about which side Gatsby actually fought on in World War I. There is a mystery surrounding whether he was an American soldier or a German spy.
"I don’t think it’s so much THAT,” argued Lucille sceptically; “it’s more that he was a German spy during the war.” One of the men nodded in confirmation. “I heard that from a man who knew all about him, grew up with him in Germany,” he assured us positively. “Oh, no,” said the first girl, “it couldn’t be that, because he was in the American army during the war.” As our credulity switched back to her she leaned forward with enthusiasm. “You look at him sometimes when he thinks nobody’s looking at him. I’ll bet he killed a man.” (Fitzgerald)
Gatsby is a difficult man to actually know, part of the reason for this is because he recreated himself, he became Jay Gatsby. He was born James Gatz from North Dakota who endured a poor upbringing by dreaming to be rich.
Another clue to Gatsby military service is the fact that he was stationed in Louisville before being shipped overseas, that is when he met Daisy Buchanan the woman who inspired him to seek and acquire wealth.
In the first conversation between Nick and Gatsby, Gatsby says, "Your face is familiar....Weren't you in the Third Division during the War?" (51). Nick says he was, explaining which battalion, and Gatsby responds,"I was in the Seventh Infantry until June nineteen-eighteen (52). If Gatsby is to be believed, this means he was in the US Army. Of course, there are so many versions of Gatsby's life floating around, including those he has provided himself, it is difficult for the reader to be sure about the truth of anything about Gatsby. As others have noted, he is a self-made man, and that means his background should be unknown. If our pasts follow us, it is difficult for us to make ourselves into something new.
he was an officer in WWI, so he defended the allied forces.
In Chapter IV Gatsby tells Nick about accepting a commission as a first lieutenant and the battle in the Argonne Forest. He says he was promoted to be a major and that every Allied government gave him a decoration - even Montenegro. He still carried the medal on a ribbon and he showed Nick. He fought for the U.S., but it was probably Daisy he was really fighting for.