The U.S. military service founded on June 14, 1775, before the Declaration of Independence, is the United States Army.
The Second Continental Congress created the Continental Army during the American Revolutionary War. George Washington was appointed its commander in chief soon afterward, on June 19, 1775. The Army therefore predates the Declaration, which Congress adopted on July 4, 1776.
June 14 is observed as the Army’s birthday. It is also the date traditionally associated with the creation of the Continental Army, although the modern United States Army developed through later legislation and reorganizations.
The Navy and Marine Corps also originated during the Revolution, but they were established later in 1775. The Air Force is much younger, becoming a separate military service in 1947.