Key dates in the construction of the atomic bomb and the bombing of Nagasaki

1939 
Albert Einstein writes to President Franklin Roosevelt concerning the use of uranium as a new source of energy leading to creation of new, extremely powerful bombs. He also notes that the German government had stopped the sale of all uranium from Czechoslovakian mines. In slow response, Roosevelt forms a special committee to consider the military implications of atomic research. 

September 1 — War begins in Europe.

1941 
December 6 — Roosevelt authorizes the Manhattan Engineering District for the purpose of creating an atomic bomb. This would later be called the “Manhattan Project.”

December 7 — The Japanese attack Pearl Harbor.

1942 
June — J. Robert Oppenheimer is appointed director of the Manhattan Project.

December 2 — Enrico Fermi’s experiments lead to the first controlled nuclear chain reaction on a squash court beneath Stagg Field at the University of Chicago. 
 
1943 
Oppenheimer and the bomb development team are moved to a secret laboratory located at Los Alamos, New Mexico.

1945 
July 16 — The first test of the “gadget,” a plutonium bomb, at Trinity Site, White Sands Missile Range, just north of Alamogordo, New Mexico. 

August 6 — Hiroshima is bombed. At 9:04 on the morning of August 6, 1945, the Enola Gay dropped the 8,900-pound atomic weapon named  "Little Boy" on Hiroshima, leveling almost 90% of the city. 

August 9 — Nagasaki is bombed. 

August 14 — Japan announces its surrender followed by a formal surrender in Tokyo Bay on September 2.

1949 
The Soviet Union detonates its first atomic device, ending America's nuclear monopoly.