In the spring of 1945, U.S. troops in the Pacific were nearing the final stages of their “island-hopping” campaign, a strategy designed to capture smaller islands in the Pacific and set up military bases in preparation for an invasion of Japan. Though the campaign was proving successful so far, it was also extremely costly: The 36-day Battle of Iwo Jima in February and March cost the United States more than 6,000 men (Japan lost 20,000).

Okinawa, located 350 miles from Japan’s southernmost island, Kyushu, was the main island in the Ryuku chain. Much of the island, which measured some 70 miles long and seven miles wide, with 463 square miles of area, was heavily cultivated with cane fields and rice paddies. Home to some 450,000 people, Okinawa boasted a larger population than other Pacific islands. Japan had annexed the island in 1879 and attempted to “Japanize” its inhabitants, who were viewed as second-class citizens by many Japanese, including soldiers in the Imperial Army. Okinawans were ethnically diverse, with different cultures, traditions and dialect than their Japanese neighbors. In the period leading up to the U.S. invasion, some civilians were evacuated from Okinawa, but most stayed put.

On April 1—Easter Sunday—after six days of bombardment, the troops of the U.S. 10th Army, commanded by General Simon B. Buckner, began their amphibious invasion of Okinawa. General Mitsuro Ushijima, leader of the more than 100,000 Japanese forces on Okinawa, made his headquarters in the 15th-century citadel of Shuri, at the southern end of the island. Determined to defend the southern, most heavily populated section of the island, he left the shoreline relatively undefended, waiting for the Americans to come to him.

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It wasn’t until a few days into the invasion that the advancing U.S. soldiers realized the true nature of the battle they were facing. Tunnel systems connected the island’s caves, and Japanese machine gunners positioned themselves in hidden stone funeral vaults dotting the hills. The Japanese mounted few attacks themselves, conserving all their fire for defending their positions against American infantry advances.

As U.S. troops on Okinawa confronted such challenges, Japanese pilots began a barrage of kamikaze attacks on the U.S. Navy’s Fifth Fleet, waiting offshore in support of the invasion. Japan’s giant battleship Yamato even made its own suicide mission, attacking the U.S. fleet on April 7 accompanied by a light cruiser and eight destroyers. Struck by a wave of Allied torpedoes and bombs, Yamato blew up and sank, along with the light cruiser Yahagi, taking with them thousands of Japanese sailors.

Despite such spectacular gestures of futility, the kamikaze tactics used by Japan at Okinawa handed the U.S. Navy their worst losses of World War II. The U.S. fleet in the Pacific had experienced Japanese suicide attacks before, but never on such a scale. By the end of the Okinawa campaign, some 1,465 kamikaze pilots sank 29 U.S. ships and damaged 120 more, killing more than 3,000 sailors and wounding another 6,000 or so more.

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By mid-May, U.S. forces had pushed Ushijima’s 32nd Army south to its final line of defenses at Mabuni. Hordes of civilians, whom Japanese soldiers terrified with tales of the brutality of U.S. troops, desperately followed the retreating army, often getting caught in the crossfire. Over some 10 days in mid- to late May, several regiments of U.S. Marines fought to secure Sugar Loaf Hill, a mound of earth barely 50 feet high and some 300 yards long, located on southern Okinawa. Concealed in a network of caves and tunnels with disguised firing positions, the Japanese troops defending Sugar Loaf were able to take out the tanks used to support the advancing Marines with mines, artillery and antitank fire. At the same time, their own positions were difficult to attack due to their camouflage. Many of the Marines who fought at Sugar Loaf never saw the enemy soldiers they faced. They finally secured the hill on May 18, after suffering some 2,662 casualties.

Forced to withdraw from Shuri Castle, Ushijima’s army had been reduced to some 30,000 men, and the battle was drawing to a close. Heavy losses still lay ahead for both sides. On June 18, General Buckner himself was killed by shell splinters while watching an attack by the Second Marine Division. Four days later, as defeat loomed, Ushijima and his subordinate, Lieutenant General Isamu Cho, committed ritual suicide in their command bunker at Mabuni.

The Battle of Okinawa stands as the costliest engagement of World War II in the Pacific. Japan lost as many as 100,000 soldiers, while the Allies suffered 65,000 casualties, including 14,000 dead. Civilians would bear the highest death toll of the battle for Okinawa, with some 100,000—and perhaps as many as 150,000—perishing during the campaign. Such imposing casualty figures, as well as the desperate nature of Japan’s kamikaze tactics, would be on the minds of U.S. commanders as they contemplated an invasion of Japan’s home islands, and ultimately made the decision to deploy atomic bombs against Hiroshima and Nagasaki.

After Japan’s unconditional surrender ended World War II, U.S. forces occupied and administered Okinawa. The ongoing presence of military bases on the island since then has been a source of controversy. During the Vietnam War, Okinawans protested the U.S. military’s use of the island as a platform to bomb Vietnam. Administration of the island reverted to Japan in 1972, but the controversial U.S. presence remains: Okinawa now houses several thousand U.S. military personnel and a dozen installations.