Ernest J. King in the context of Combined Chiefs of Staff

The military history of the United States during World War II covers the nation's role as one of the major Allies in their victory over the Axis powers. The United States is generally considered to have entered the conflict with the 7 December 1941 surprise attack on Pearl Harbor by Japan and exited it with the surrender of Japan on 2 September 1945. During the first two years of World War II, the U.S. maintained formal neutrality, which was officially announced in the Quarantine Speech delivered by President Franklin D. Roosevelt in 1937. While officially neutral, the U.S. supplied Britain, the Soviet Union, and China with war materiel through the Lend-Lease Act signed into law on 11 March 1941, and deployed the U.S. military to replace the British forces stationed in Iceland. Following the 4 September 1941 Greer incident involving a German submarine, Roosevelt publicly confirmed a "shoot on sight" order on 11 September, effectively declaring naval war on Germany and Italy in the Battle of the Atlantic. In the Pacific Theater, there was unofficial early US combat activity such as the Flying Tigers.

During the war, some 16,112,566 Americans served in the United States Armed Forces, with 407,316 killed and 671,278 wounded. According to the US Department of Defense, of the 407,316 dead, about 250,000 were killed in the European theater, the remaining 160,000 died in the Pacific War. There were also 130,201 American prisoners of war, of whom 116,129 returned home after the war. Key civilian advisors to President Roosevelt included Secretary of War Henry L. Stimson, who mobilized the nation's industries and induction centers to supply the U.S. Army, commanded by General George C. Marshall and the Army Air Forces under General Henry H. Arnold. The U.S. Navy, led by Secretary of the Navy Frank Knox and Admiral Ernest J. King, proved more autonomous. Overall priorities were set by Roosevelt and the Joint Chiefs of Staff, chaired by William D. Leahy. The defeat of the Nazis was the U.S.'s official highest priority per its agreement with Britain; however, in practice, the US devoted more resources to the Pacific than Europe and Africa until 1944.

Fleet admiral (abbreviated FADM) is a five-star flag officer rank in the United States Navy whose rewards uniquely include active duty pay for life. Fleet admiral ranks immediately above admiral and is equivalent to General of the Army and General of the Air Force. It is the same as the discontinued Admiral of the Navy.

Although it is a current and authorized rank, no U.S. Navy officer holds it presently. Only four officers have ever held the rank, all in the World War II era: William D. Leahy, Ernest J. King, Chester W. Nimitz, and William Halsey Jr. Leahy, King, and Nimitz were promoted to the rank in December 1944, followed by Halsey in December 1945. While all four men effectively retired in the late 1940s, the rank of fleet admiral is for life. The last living fleet admiral was Nimitz, who died in 1966.

William Frederick "Bull" Halsey Jr. (30 October 1882 – 16 August 1959) was an American Navy admiral during World War II. He is one of four officers to have attained the rank of five-star fleet admiral of the United States Navy, the others being William Leahy, Ernest J. King, and Chester W. Nimitz.

Born in Elizabeth, New Jersey, Halsey graduated from the United States Naval Academy in 1904. He served in the Great White Fleet and, during World War I, commanded the destroyer USS Shaw. He took command of the aircraft carrier USS Saratoga in 1935 after completing a course in naval aviation, and was promoted to the rank of rear admiral in 1938. At the start of the War in the Pacific (1941–1945), Halsey commanded the task force centered on the carrier USS Enterprise in a series of raids against Japanese-held targets.