The United States Army is the largest branch of the United States Armed Forces and, like all branches, comes under the ultimate command of the President of the United States, the Commander in Chief. The United States Army joined the war in the Pacific in 1941 after the Pearl Harbor attack. The United States Army participated in every theater except the Eastern European front.
On 2 March 1942, all branches of the US Army were replaced by three autonomous commands - Air Forces, Ground Forces and Service of Supply. All three commands were the direct responsibility of the General Staff.