The Manhattan Project is the code name for the US government's secret project that was established before World War II and culminated in the development of the nuclear bomb. The idea of forming a research team to create a nuclear weapon was endorsed in a letter than Einstein sent to Franklin Roosevelt, the president of America at the time. This was in 1939. In 1942 Enrico Fermi, a physicist, successfully controlled a nuclear reaction in his reactor called CP-1 (Chicago Pile 1). CP-1 was located at the University of Chicago under a squash court, quite incredibly. The following was said by a member of the project:

Later in the project the first atomic bomb was exploded at White Sands. This was on July 16, 1945. The director of Los Alamos said upon witnessing the first test of a nuclear weapon:

We knew the world would not be the same. A few people laughed, a few people cried, most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita. Vishnu is trying to persuade the Prince that he should do his duty and to impress him takes on his multi-armed form and says, "Now, I am become Death, the destroyer of worlds." I suppose we all felt that one way or another.
J. Robert Oppenheimer

Bomb Dropped on Nagasaki
Picture of a Temporary Fuel Rod Storage Pool
Original Source: National Archives

A month after the first bomb was tested, two nuclear weapons were exploded over Japan, at Hiroshima and Nagasaki. There were many reasons for this. The official reason is that it would immediately end the war, thus saving the lives of thousands of American servicemen. Immediate deaths from the bomb are estimated to be about 100,000. This figure is astounding. However, it is comparable to the estimated number of casulaties that would have resulted from a Allied invasion of the Japanese home islands. However, the choice to drop the bombs on Japan is very controversial and there are many people that feel they were unneccsary, and that Japan would have surrenedered anyway.

Undoubtedly, the atomic bomb is the most powerful destructive force that mankind has ever wielded. However, many scientists defend their participation in it's creation:

At Los Alamos during World War II there was no moral issue with respect to working on the atomic bomb. Everyone was agreed on the necessity of stopping Hitler and the Japanese from destroying the free world. It was not an academic question ‚ our friends and relatives were being killed and we, ourselves, were desperately afraid.
-Joseph O. Hirschfelder, chemist

At Los Alamos we had some conversations on the subject and I must admit that my own position was that the atom bomb is no worse than the fire raids which our B-29s were doing daily in Japan, and anything to end the war quickly was the thing to do.
-George B. Kistiakowsky

First Fusion Bomb Test
This test was conducted on an island. After the bomb went off, the entire island was gone.
First H-Bomb Explosion
Original Source: DOE
Soon after the Manhattan Project became a success, the Soviet Union developed their own atomic bomb. With these new weapons that could destroy entire cities and civilizations, the atomic arms race and cold war began. First these weapons were attached to bombers, but soon space-based systems were developed. These systems utilized rockets that could take a nuclear warhead from one side of the globe to another in under an hour. Since the 1950's, there has existed an amount of weaponry on earth great enough to destroy humanity and all it's accomplishments. No one has used these weapons against another country since WW-II, however. This is because of the concept known as Mutally Assured Destruction (MAD). If the US were to launch an attack against Russia, the Russians would immediately retaliate by launching their missiles, and not only would the US be destroyed, but Russia would as well. Thus, in this game of nuclear warfare, no one can win. If either side decides to play, they both lose, as well as the rest of the world.