In mid-1944, Oppenheimer shifted the focus of work at Los Alamos from research to the design of the two bombs, Fat Man and Little Boy. It was an indication that the project had reached a new stage–the scientists moving from theory to application–and they began to prepare themselves for the first test of their work.

It was decided that Little Boy, the U-235 bomb, needed no test, as its design was basic enough that scientists were certain it would work. But the plutonium bomb, Fat Boy, was less of a sure thing. Oppenheimer and Groves decided to arrange a test, which would be the world's first nuclear explosion.

They selected an isolated area in the middle of the desert near Alamogordo, New Mexico, called the Jornada del Muerto, or "Journey of Death." The desolate land stretched sixty miles to the north and south and forty miles to the east and west. It was code-named "Trinity."

No one knows who named the site, but some have claimed that the name came from Oppenheimer himself. According to this version of the story, Oppenheimer took the idea from a John Donne poem, which read, "Batter my heart, three-personed God; for you / As yet but knock, breathe, shine and seek to mend; / That I may rise, and stand, o'erthrow me, and bend / Your force, to break, blow, burn, and make me new."

There was initial concern about what would happen after the explosion; the scientists realized that they could only complete the test if the wind was traveling in a certain direction. If it was blowing the wrong way, the wind could carry fallout (radioactive dust and debris) over civilized areas. The lead physicists, particularly Oppenheimer, were consumed by anxiety as they waited to see if the test could take place and, more crucially, if the bomb would work.

The wait seemed to last forever.

Then, finally, they got the go ahead. The Trinity test could take place. Everything would proceed as planned, and the physicists, technicians, military, and government observers assembled themselves a safe distance away. And waited.

On Monday, July 16, 1945, 5:29:45 a.m., the bomb exploded.

The flash of light was seen in three states. The mushroom cloud rose 38,000 feet high. The explosion itself created a half-mile wide crater in the earth, the heat of the bomb fusing the desert sand into glass. William Laurence, the only journalist allowed on the site, reported, "One felt as though he had been privileged to witness the Birth of the World–to be present at the moment of Creation when the Lord said: 'Let There Be Light.'"

Among the physicists, there was applause and cheering, a sense of triumph and relief. The gadget had worked! Oppenheimer was less than enthused. As the bomb exploded, the man perhaps most responsible for its creation instantly thought of a line from the ancient Hindu text the Bhagavad Gita: "I am become Death / The destroyer of worlds."

Soon after, Oppenheimer joined in the general celebration, as everyone was thrilled that the device had worked. It was only after the initial relief wore off that the scientists began to have their own second thoughts, realizing what their hard work had created: the most effective killing machine known to man.

Trinity was more than a demonstration of the bomb: for some, it was a fearful reminder of what nuclear power might be capable of; for others, however, it was the triumphant realization of years of work and over two billion dollars of government money. The time had come to decide for what purpose all those years and all those dollars had served. The time had come to decide whether or not to use the bomb, not in a demonstration, but in an act of war.

President Harry Truman endlessly debated the question with his advisors–should the United States use the bomb against Japan? The only other option, Groves argued, was to invade the country, a mission that threatened the loss of over one million men. While not everyone agreed that the losses would be this high, they did agree that even 35,000 losses (a more conservative estimate) were too many to risk.

A compromise was proposed: perhaps the United States could give Japanese leaders a demonstration of the bomb's power in an uninhabited area. Surely, these reluctant physicists and politicians argued, this would be enough to convince the Japanese that they should surrender. But Oppenheimer–as well as Groves and Truman–objected to this plan, afraid that the Japanese would refuse to back down, and the element of surprise would be lost.

Eventually Truman decided that the bomb would be used against Japan. In fact, historian Martin J. Sherwin has argued that the issue was never seriously in question and that it was always assumed that the atomic bomb would be used to end the war. Oppenheimer agreed, noting, "the decision was implied in the project."

On August 6, 1945, a B-29 bomber, the Enola Gay, dropped the Little Boy bomb on the city of Hiroshima, population 400,000. The city had been evacuated in anticipation of attack, leaving only 300,000 residents – the majority of whom were civilians. In a single instant, the bomb destroyed 70,000 buildings–over ninety percent of the city's structures. One hundred thousand people died instantly, or within a couple days of the explosion. At least 100,000 more died within five years. Three days later, on August 9, the United States dropped a second bomb, the Fat Man. This bomb was dropped on the Japanese city of Nagasaki, and it killed another 100,000 people.

Five days later, the Japanese Emperor Hirohito announced his surrender.

The majority of the country supported the dropping of the bomb on Japan, as did the majority of the scientists who worked on the project. Oppenheimer was initially thrilled to hear that the bombs had worked, and it was only later that he began to fear the consequences of a nuclear world.

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