What is the only branch of the military specifically authorized to maintain a standing force under the constitution?

Authorized.....unauthorized.....I'll still sleep safe and sound. "You cannot invade the mainland United States. There would be a rifle behind every blade of grass." Admiral Yamamoto (although this quote fact is often challenged).

NAVY and until the late 1940's reported directly to the President. General Marshall engineered the Admiral's surrender and brought every military organization under the control of the Department of Defense.

Due to use of armies by tyrants congress felt that a navy would less likely be a threat to the new nation. Also the ability to have safe waters was paramount for the economy.