The Manhattan Project was the government project that took place from 1942 to 1946, the purpose of which was to develop a nuclear bomb. It succeeded on 16 July 1945 at the Trinity Test in New Mexico and went on to produce the two atomic bombs that destroyed the Japanese cities of Hiroshima and Nagasaki during WWII. Controlled by the U.S. Army Corps of Engineers, under the leadership of J. Robert Oppenheimer, the Manhattan Project is considered one of the most successful crash science/engineering projects of all time. Its success had huge implications for the Pacific Theater of World War II and the specter of nuclear war that has existed ever since. The project grew to employ 130,000 people working at secret locations, and cost $1.9 billion US Dollars (in 1946 figures; this is about $23.5 billion USD in 2012).
The Manhattan Project began with a letter written by Albert Einstein and Leó Szilárd to US President Franklin D. Roosevelt, who argued that the Nazis could develop nuclear bombs that they could use to win the war. Szilárd had discovered in 1933 that a nuclear chain reaction could be self-sustaining, and kept the result secret for as long as he could, fearful that fascist governments would exploit the technology to make nuclear bombs.
In March 1941, scientists determined that a nuclear bomb could be created with only 25 pounds (11.3 kg) of Uranium-235, far less than physicists expected, and that a bomb could be created within a reasonable amount of time for use during the war. A program was launched by the US government, and by the spring of 1942, a large facility was constructed at Los Alamos, New Mexico. The best nuclear physicists in the country at the time gathered at this location, which was chosen for its remoteness. Oak Ridge, Tennessee, and Hanford, Washington, were chosen as the primary sites of uranium isotope separation, which was made easier by the abundant availability of cheap hydroelectric power.
Many of the best scientists which contributed to the Manhattan project were Jewish emigres who came from Europe to escape Hitler. For example, Franz Simon and Nicholas Kurti came up with a way to separate uranium-235 from uranium ore, which was necessary for completion of the project. One of the two atomic bombs used in the war was actually made from plutonium, which at the time was considered a very exotic element. After the successful nuclear test at Alamogordo, New Mexico, it was only a matter of time before the government decided to put the weapon to use in warfare.