The Battle of Peleliu, codenamed Operation Stalemate II by the US military, was fought between the United States and Japan during the Mariana and Palau Campaign of World War II, from September 15 to November 27, 1944, on the island of Peleliu.

US Marines of the 1st Marine Division and then soldiers of the US Army's 81st Infantry Division, fought to capture an airstrip on the small coral island of Peleliu. The battle was part of a larger offensive campaign known as Operation Forager, which ran from June to November 1944 in the Pacific Theater.

Major General William Rupertus, the commander of the 1st Marine Division, predicted that the island would be secured within four days. However, after repeated Imperial Japanese Army defeats in previous island campaigns, Japan had developed new island-defense tactics and well-crafted fortifications, which allowed stiff resistance and extended the battle to more than two months. The heavily-outnumbered Japanese defenders put up such stiff resistance, often by fighting to the death in the Japanese Emperor's name, that the island became known in Japanese as the "Emperor's Island."In the US, it was a controversial battle because of the island's negligible strategic value and the high casualty rate, which exceeded that of all other amphibious operations during the Pacific War. The National Museum of the Marine Corps called it "the bitterest battle of the war for the Marines".