Since when was a President allowed to create his own Department in our Government?

[kentuck]

5. From Wikipedia:

"The United States Atomic Energy Commission (AEC) was an agency of the United States government established after World War II by the U.S. Congress to foster and control the peacetime development of atomic science and technology.[2] President Harry S. Truman signed the McMahon/Atomic Energy Act on August 1, 1946, transferring the control of atomic energy from military to civilian hands, effective on January 1, 1947.[3] This shift gave the members of the AEC complete control of the plants, laboratories, equipment, and personnel assembled during the war to produce the atomic bomb.[4]"