Two pivotal events in July 1945 slammed the door on Japanese hopes for negotiating a favorable settlement to end the war. Allied leaders meeting in Potsdam, Germany to discuss postwar plans in Europe issued a declaration demanding Japan’s unconditional surrender, and the United States detonated a test device in the New Mexico desert, producing the very first atomic explosion.

For months, military leaders had been pouring over the grim details for Downfall, the plan for invading the Japanese home islands. With a force of 1.2 million soldiers and Marines, the two-phased operation would begin with an invasion of Kyushu, the southernmost island, followed months later by an invasion of the main island of Honshu, home to Tokyo, the Imperial Palace, and most of the Japanese population. It would be an epic contest for both islands, and the scale of likely casualties and destruction was jaw-dropping. The number of Americans killed and wounded on Kyushu and Honshu was expected to reach into the hundreds of thousands, and the number of Japanese dead would be in the millions.

Even at this late stage of the war, the Japanese were not without power. Despite three years of defeats, they still had millions of men in uniform, and though most remained tied down on the Asian mainland or stranded on Pacific outposts, there were still sizable armies in Japan prepared to defend their sacred soil. A citizen army comprising millions of Japanese men, women, and even children was also expected to answer a call to arms. With weapons and ammunition in short supply, most would be armed with little more than bamboo spears, assuring their certain deaths against an onslaught of American firepower.

Unwilling to accept such staggering human losses, President Harry S. Truman approved the use of atomic weapons to compel a final surrender. The first target was Hiroshima, an industrial city and the headquarters for the army defending Kyushu. On the morning of August 6, a B-29 Superfortress dropped a uranium-fueled bomb from an altitude of 31,000 feet – nearly six miles. Detonating a half-mile above ground with an explosive force of 15,000 tons of TNT, the blast killed 70,000 people and injured another 70,000, most with severe burns. Tens of thousands of others outside the blast radius would later die from radiation poisoning.

Still, Japanese officials remained defiant. On August 8, the Soviets finally abandoned their longtime neutrality and attacked Imperial Army forces on the Asian mainland with more than one million men. The next day, a second atomic bomb, this one fueled with plutonium, was dropped on the port city of Nagasaki, leading to another 140,000 deaths. Emperor Hirohito’s war cabinet remained deadlocked, but the emperor could no longer allow his people to suffer. He broke the tie among his war ministers, and his voice soon filled the radio airwaves nationwide, informing a war-weary population that he had agreed to the Allied terms.

On September 2, 1945, Japanese dignitaries boarded the battleship USS Missouri in Tokyo Bay. With General MacArthur presiding over the surrender ceremony and Admiral Nimitz serving as the signatory for the United States, the war was formally brought to a close.