The term "European Theater of Operations" should not be confused with the European Theatre of World War II which is often defined to include the years before the US entered the war, the Italian campaign, the European Strategic Bombing Campaign, the European Eastern Front, all of the European Western Front in 1944 and 1945, as well and other actions which did not involve the use of American forces.
Operation Torch, the landings in North Africa, were referred to as occurring in the North African Theater of Operations and then later (December 10, 1944), when the theater was redefined to include Italy, as the Mediterranean Theater of Operations (MTO). US forces in that theatre were initially under the administrative command of NATOUSA which was redesignated MTOUSA. They were under the operational command of Allied Forces Headquarters (AFHQ).
Five months later, the United States Department of War officially established ETOUSA, on June 8, 1942. Its mission was to conduct planning for the eventual retaking of Europe and to exercise administrative and operational control over U.S. forces. Headquartered in London, ETOUSA was first commanded by Major-General James E. Chaney, an Army Air Corps officer.
U.S. General Dwight D. Eisenhower had multiple command appointments; he was replaced by Chaney in late June 1942, but in November he also commanded the Allied forces in Operation Torch through AFHQ. He then gave up command of ETOUSA in February 1943 to be NATOUSA. In December 1943 it was announced that Eisenhower would be Supreme Allied Commander in Europe. In January 1944 he resumed command of ETOUSA and the following month was officially designated as the Supreme Commander of the Allied Expeditionary Forces. (Note that Supreme Headquarters of the Allied Expeditionary Forces (SHAEF) was the headquarters of the Commander of the Allied Expeditionary Forces, whereas the AFHQ was the headquarters of only the Allied forces). He served in a dual role until the end of hostilities in Europe in May 1945. From February 1944, SHAEF was the operational command and ETOUSA administrative command.
Some units were transferred between operational commands and administrative commands at different times. For example the American 6th Army Group, which was set up under the Mediterranean Theater of Operations to oversee Operation Dragoon, the invasion of southern France between Toulon and Cannes, was passed to SHAEF (and into ETO) a month after the invasion which took place on August 15 1944.
By the end of 1944, Eisenhower, through SHAEF, commanded three powerful Allied army groups. In the North British 21st Army Group commanded by Field Marshal Sir Bernard Montgomery, In the middle the American 12th Army Group commanded by General Omar Bradley and in the South the American 6th Army Group commanded by Lieutenant General Jacob L. Devers. The British 21st Army Group and French elements of the 6th Army Group were not part of ETOUSA, but by that stage of the war most of the operational forces under the command of SHAEF were American.
Albert Coady Wedemeyer was chief author of the Victory Program, published three months before the U.S. entered the war in 1941, which advocated the defeat of the German armies on the European continent. When the U.S. entered the war after the Japanese bombed Pearl Harbor on 7 December 1941 and the U.S. was at war with both Japan and Germany, a modified version of his plan was adopted by U.S. President Franklin D. Roosevelt and British Prime Minister Winston Churchill. Under the German first policy, the plan was expanded to include the blue print for the Normandy landings.
Until SHAEF was operational ETOUSA liaised closely with the British in the planning and organising of Operation Overlord.