The oldest military branch of the United States is the US Army, founded in 1775. There are approximately 571,108 full-time and active soldiers in today's US Army, serving worldwide.
The United States Military today is made up of five active-duty services and their respective Reserve and Guard units. The United States Marine Corps was also founded in 1775, as was the Navy.
The US Air Force had its beginnings as a subdivision of the Army. In 1920, the Air Force was declared an official combatant arm of the military. Following World War II, in 1947 the Air Force was finally officially recognized as its own military branch.
The US Coast Guard was the result of the government combining (initially) five separate Federal services. In 1915, a congressional act combined two of those Federal services to form the Coast Guard. The Coast Guard operated under the control of the Treasury Department until 1967, when it was transferred to the newly formed Department of Transportation by executive order.
At present, during peacetime the Coast Guard operates under the Department of Homeland Security, and under the Navy during wartime (or by special presidential order).