By the time they splashed their way onto its southeastern beach on February 19, 1945, many of the U.S. Marine invasion force wondered if there were any Japanese left alive on Iwo Jima.

Allied aircraft, battleships and cruisers had spent the previous two and a half months pulverizing the volcanic outcropping with thousands of tons of high explosives, leaving it a smoldering heap of charred boulders and burned-out vegetation. A haze of smoke now covered much of the island, and the stench of cordite and sulphur hung heavy in the air. “There wasn’t a tree left standing,” Corporal Stacy Looney later remembered, “wasn’t anything left standing.”

The Marines had been told to expect heavy resistance, but the first waves of landing craft encountered only a few artillery bursts and scattered small arms fire. Thousands of infantrymen, tanks and vehicles were able hit the beach with relative ease.

“There’s something screwy,” one corporal said of the ominous calm. The Marines were right to be suspicious. As soon as the first units advanced onto an ash-covered terrace beyond the shore, dozens of camouflaged Japanese batteries erupted with murderous mortar and machine gun fire, and artillery shells began raining down on the men and equipment still clogging the beach. “The honeymoon is over!” one officer yelled. In an instant, any illusions the Marines had of taking the island without a fight had evaporated.

Outside of its proximity to Japan—still some 750 miles away—the 8 square mile hunk of land at Iwo Jima carried little significance. It lacked adequate supplies of fresh water and other resources, and its shores were too rocky to act as harbors for Navy ships.

But as World War II moved closer to its conclusion, the island had become a crucial steppingstone in the American push toward the Japanese homeland. B-29 Superfortresses had begun making bombing runs over Tokyo, and they needed Iwo Jima as an emergency landing site and staging ground for their fighter escorts. To seize the island, the U.S. high command had marshaled the 3rd, 4th and 5th Marine divisions of the V Amphibious Corps under Lt. General Holland “Howlin’ Mad” Smith. The total force included a staggering 70,000 men—the most Marines ever assembled for a single operation.

Standing in the way of the American invasion were some 22,000 Japanese led by General Tadamichi Kuribayashi. Under his leadership, Iwo Jima’s garrison had transformed the island into a labyrinth of natural caves, subterranean tunnels and fortified pillboxes and bombproofs. Nearly all of the Japanese emplacements contained a copy of a special order from Kuribayashi commanding his men to fight to the bitter end.

“Above all, we shall dedicate ourselves and our entire strength to the defense of this island,” the instructions read. “Each man will make it his duty to kill ten of the enemy before dying.” Thanks to their stout defenses, Kuribayashi’s men had suffered surprisingly few casualties during the American artillery onslaught. When the Marines finally moved past the beach on the morning of February 19, they sat waiting with guns trained.

Once the shooting started, the American landing zone turned into a cauldron of shell bursts and mortar fire. Thomas McPhatter, one of several hundred African American Marines who joined in the attack as amphibious truck drivers and ammunition handlers, later described the hellish scene to the Guardian.

“I jumped in a foxhole and there was a young white Marine holding his family pictures,” he said. “He had been hit by shrapnel, he was bleeding from the ears, nose and mouth. It frightened me. The only thing I could do was lie there and repeat the Lord’s prayer, over and over and over.”

After braving the intense fire, U.S. troops established a beachhead and began knocking out Japanese pillboxes and trenches near the shoreline. Others made a stubborn slog through foot-deep volcanic ash and crossed to the island’s western side, cutting off its 550-foot-tall southern peak at Mt. Suribachi. By nightfall, more than 30,000 Marines had landed on Iwo Jima.

U.S. forces continued their advance over the next several days, capturing the first of three airfields and moving toward the island’s rock-strewn northern sector. On February 23, elements of the 28th Marines took the heights at Suribachi to the sound of cheers and celebratory gunfire from the men watching below.

Associated Press Photographer Joe Rosenthal snapped a now-famous photo of six Marines struggling to hoist the Stars and Stripes atop the mountain, yet the flag raising was only a brief moment of triumph in what had become a bitter battle. Marines would continue fighting for another month through hills and gullies with nicknames like the “Meat Grinder,” “Death Valley” and “Bloody Gorge,” suffering thousands of casualties for every mile of territory gained.

Fighting at Iwo Jima often took the shape of a deadly game of cat and mouse. General Kuribayashi had dispensed with the costly “banzai” charges typically practiced by the Japanese army and ordered his men to fight in a fashion that more closely resembled guerilla warfare. Japanese troops would ambush Marines and then disappear into their warren of caves and tunnels, only to reappear in new positions. “At great cost, you’d take a hill to find then the same enemy suddenly on your flank or rear,” said Fred Haynes, then a captain. “The Japanese were not on Iwo Jima. They were in it!”

Small arms fire proved futile against the Japanese pillboxes and tunnels, so Marines relied on their M2 flamethrowers, bazookas and fire-spewing Sherman “Zippo” tanks to clear out enemy fortifications. Grenades became the soldiers’ most handy weapons, with both sides rolling them down hills and tossing them into caves. While administering first aid to wounded men, one Navy medic named John Harlan Willis retrieved and threw back eight Japanese grenades before the ninth exploded in his hand and killed him. He was posthumously awarded the Medal of Honor.

By early March, battle-weary Marines had captured Iwo Jima’s two remaining airfields and reached the northern shoreline, effectively splitting the island in half. The surviving Japanese troops were severely outnumbered, and many had gone days without water. Nevertheless, very few surrendered.

“They never had any sort of sustenance compared to what our Marines had,” Colonel John Ripley later said of the Japanese, “but at the same time they fought and fought and fought, and what a hell of a job they did.”

As the battle wore down, the remnant of Kuribayashi’s forces moved through the island like ghosts, donning captured U.S. uniforms and launching surprise nighttime counterattacks. “It’s like fighting something abstract and intangible,” one American lieutenant complained. “We’d be glad to fight these people if we could only see them.” Japanese resistance continued long after the island was deemed secure, culminating in a desperate final assault on March 26. Later that same day, Marine Corps brass finally declared an official end to combat operations on Iwo Jima.

The five-week long campaign had taken a bitter toll on the American invasion force, which left the island with nearly 7,000 Marines and Navy men dead and another 20,000 wounded. President Roosevelt reportedly gasped when he heard the numbers. The Japanese, most of whom obeyed their orders to fight to the last, lost around 21,000 men.

Among the dead was Kuribayashi, who either perished in combat or committed suicide. The remaining Japanese surrendered or were taken prisoner, but a few holdouts disappeared into Iwo Jima’s underground hive of caves and tunnels. The last two Japanese on the island only surrendered in 1949—a full four years after the war had ended.

Iwo Jima went on to save countless American lives as an emergency landing strip for Air Force bombers in the Pacific, but any larger role it might have played in an invasion of Japan was made irrelevant after the atomic bomb fell over Hiroshima and Nagasaki in August 1945. Nevertheless, the battle for the small, barren island continued to loom large in the American consciousness, both for Rosenthal’s iconic photo of the flag raising on Mt. Suribachi and for the legendary grit of the Marines and Navy men who fought on in the face of overwhelming misery.

“Victory was never in doubt…its cost was,” 3rd Marine Division leader Graves B. Erskine later said of Iwo Jima. “What was in doubt, in all our minds, was whether there would be any of us left to dedicate our cemetery at the end, or whether the last Marine would die knocking out the last Japanese gun and gunner.”