Right now the top rank someone can achieve in the U.S. military is "O-10" (four star general or admiral), but there are ranks that aren't in service right now that have existed and may exist again in the future. I find this fascinating.
So, the 5 star rank only exists in times of war.
Five-star ranks were created in the U.S. military during World War II because of the awkward situation created when some American senior commanders were placed in positions commanding allied officers of higher rank. U.S. officers holding five-star rank never retire; they draw full active duty pay for life. The five-star ranks were retired in 1981 on the death of General Omar Bradley.
Nine Americans have been promoted to five-star rank, one of them, Henry H. Arnold, in two services.
As part of the bicentennial celebration, George Washington was, 177 years after his death, permanently made superior to any other five-star general/admiral with the title general of the armies effective on July 4, 1776.
Fleet admiral of the United States Navy, or more commonly referred to as fleet admiral (FADM), is a five-star flag officer rank, and it is considered to be the highest possible rank attainable in the United States Navy. Fleet admiral ranks immediately above admiral and is equivalent to general of the army and general of the Air Force. The fleet admiral rank is reserved for wartime use only and the grade is not currently active.
A special grade of admiral of the Navy, which ranks above fleet admiral, was once conferred to Admiral George Dewey following the Spanish–American War (1898) in 1903, but it ceased to exist after his death on 16 January 1917.
The insignia for a fleet admiral is composed of five silver stars in a pentagonal design. Worn on the service dress uniform sleeve is a two-inch rank stripe, then four half inch stripes, and then a single five-pointed star, point down.
In keeping with a tradition dating back to the 18th-century Royal Navy, a fleet admiral is entitled to full admiral's pay and fringe benefits, including a small staff, for the remainder of his life.