The Battle of Peleliu, codenamed Operation Stalemate II, was fought between the United States and the Empire of Japan in the Pacific Theater of World War II, from September–November 1944 on the island of Peleliu, present-day Palau. First Division Marines and later Army soldiers of the 81st Infantry Division, fought to capture an airstrip on the small coral island.

Major General William Rupertus—commander of 1st Marine Division—predicted the island would be secured within four days. However, due to Japan's well-crafted fortifications and stiff resistance, the battle lasted over two months. In the United States, it was a controversial battle because of the island's questionable strategic value and the high casualty rate, which was the highest for U.S. soldiers of any battle in the Pacific War. The National Museum of the Marine Corps called it "the bitterest battle of the war for the Marines".

By the summer of 1944, victories in the Southwest and Central Pacific had brought the war closer to Japan, with American bombers able to strike at the Japanese main islands from air bases secured during the Mariana Islands campaign (June—August 1944). There was disagreement among the U.S. Joint Chiefs over two proposed strategies to defeat the Japanese Empire. The strategy proposed by General Douglas MacArthur called for the recapture of the Philippines, followed by the capture of Okinawa, then an attack on the Japanese mainland. Admiral Chester Nimitz favored a more direct strategy of bypassing the Philippines, but seizing Okinawa and Taiwan as staging areas to an attack on the Chinese mainland, followed by the future invasion of Japan's southernmost islands. Both strategies included the invasion of Peleliu, but for different reasons.

Peleliu (or Beliliou) is an island in the island nation of Palau. Peleliu forms, along with two small islands to its northeast, one of the sixteen states of Palau. It is located northeast of Angaur and southwest of Koror. Peleliu has a total area of 5 mi² (13 km²) As of 2004^[update], its population was about 700, making it the third most populous state of Palau. Most of the island's population lives in the village of Kloulklubed which is the state capital on the northern coast. Including the capital, there are a total of four villages:

The island was the site of the Battle of Peleliu in World War II. Peleliu is a memorial site for both WWII U.S. and Japanese troops. Many Marines died on the beaches and in the caves of Peleliu. Many of the military installations of the era, such as the airstrip, are still intact, and shipwrecks from the battle remain visible underwater just off the coast.

The battle for the island was particularly brutal because by this time the Japanese military had evolved island defense tactics to those of "endurance (fukakku) engagements." Avoiding decisive battles which characterized earlier island campaigns and often included the dramatic banzai attack, Japanese commanders instead deployed their soldiers to caves and other dug-in positions which had to be taken one by one—maximizing casualties on the attacking force. Peleliu's terrain proved ideal for such tactics, with areas of rock and cave formations heavily favoring the defenders.