Across the course of The Simpsons, a handful of character histories have been disclosed, including Abe Simpson's time fighting for American in the military. However, the show has gone back-and-forth on what he actually did while enlisted.
The Simpsons has joked that he served in every branch of the military at some point or another, but while he's mentioned being in the Coast Guard and the Marines, there's actual evidence the character served in the Army, the Navy, and the Air Force as well — and that all three changed his life forever.
One of Abe Simpson's most consistent character beats is his tendency to ramble on about impossible stories and absurd recollections of his past. But his military service definitely occurred, as it has had a number of serious impacts on his life and his family's. His most notable military service was with the United States Army during World War II. Assigned to the Flying Hellfish, Abe rose in the ranks to become Platoon Leader and Master Sergeant, as revealed in the Season 7 episode, "Raging Abe Simpson and His Grumbling Grandson in 'The Curse of the Flying Hellfish.'"
The platoon served in the European theater of the war, making its way through Europe after surviving D-Day (which Abe was implied to have survived through in Season 15's "The Regina Monologues"). Abe even teases that he once came close to killing Hitler in the middle of the conflict as he was inspecting the troops, only for Private Charles Montgomery Burns to throw off his shot. At the conclusion of the war, the Flying Hellfish discovered a castle full of priceless artwork, and after being convinced to steal them and take part in a tontine by Burns, hid it away from the world to collect later in life. Abe eventually returned to civilian life with the lingering memories of his wartime experience, even serving as the model for toy army men, as revealed in Season 30's "Mad About Toy."
However, Abe also seemingly served in other parts of the US military, apparently seeing other battles across the course of the global conflict. In "Thursdays With Abie" from Season 21, Abe tells a story about how he was in the Navy during World War II for a time, aboard the USS World War One before it was sunk in shark-infested waters. In Season 9's "Simpson Tide," he tells a story about serving on a ship that carried John F. Kennedy. Overhearing Kennedy preparing for his world-famous "Ich bin ein Berliner" speech, Abe mistook him for a Nazi sympathizer and attacked him.
The holiday themed-episode "Simpsons Christmas Stories" from Season 17 also suggested Abe was a Naval Pilot, alongside his brother Cyrus. However, the nature of that episode — including Abe saving Santa Claus from Mr. Burns — makes it likely non-canon in the flexible timeline of The Simpsons. Abe also served in the Air Force afterward, as revealed in Season 26's "Let's Go Fly A Coot." He spent time helping maintain experimental aircraft, eventually taking one for his own flight and breaking the sound barrier. Abe discussed this with a local waitress named Mona whom he formed a relationship with and later married. The two eventually had a son — Homer. It's notable that, while these stories often include embellishments, Abe did in fact serve his country.