why did they dicided to build the manhattan project?

Answer 1

One reason, and one reason only.....the allies heard that the Germans were building the nuke and we had to beat them to the punch at all costs. The 'heavy water' experiments by the Nazis were sabotaged by the the allies and the Germans didn't commit naitonal resources to complete the project.....thankfully for us.

Answer 2

This is wartime effort to build atomic bomb before Germany can create one.

Details in Wiki:
refers to the project to develop the first nuclear weapons during World War II by the United States, the United Kingdom and Canada. Formally designated as the Manhattan Engineering District (MED), it refers specifically to the period of the project from 1942-1946 under the control of the U.S. Army Corps of Engineers, under the administration of General Leslie R. Groves, with its scientific research directed by the American physicist J. Robert Oppenheimer.

The project succeeded in developing and detonating three nuclear weapons in 1945: a test detonation on July 16 (the Trinity test) near Alamogordo, New Mexico; an enriched uranium bomb code-named "Little Boy" detonated on August 6 over Hiroshima, Japan; and a plutonium bomb code-named "Fat M

Answer 3

Yea what they Said ^^
and because Einstein sent the president a letter urging him to start thinking of ways to use nuclear energy