What caused the US Army to take over atomic research in the early 1040's?

United States took over to the atomic research during the Second World War to counter the Germans effort of creating the atomic weapons.

Explanation:

To counter the Germans effort in making the nuclear technologies based weapons, America has started the Manhattan Project to discover the atomic technology related weapons.

The effort of making nuclear technology weapons was a successful and in the 1945 July 16, team of scientists and engineers have test fired the atomic bomb invented by J. Robert Oppenheimer.

They have invented two types of weapons one is atomic bomb and another is the gun type atomic weapon. To make Japanese troops surrender American president Truman ordered the atomic bombing at Hiroshima and Nagasaki.