European Theater of Operations United States Army The European Theater of Operations , United States Army ( ETOUSA ) was a United States Army formation which directed U . S . Army operations in parts of Europe from 1942 to 1945 . It referred to Army Ground Forces , United States Army Air Forces , and Army Service Forces operations north of Italy and the Mediterranean coast , in the European Theater of World War II . It was bordered to the south by the North African Theater of Operations , US Army ( NATOUSA ) , which later became the Mediterranean Theater of Operations ( MTOUSA ) . The term " theater of operations " was defined in the US Army field manuals as " the land and sea areas to be invaded or defended , including areas necessary for administrative activities incident to the military operations " . In accordance with the experience of World War I , it was usually conceived of as a large land mass over which continuous operations would take place and was divided into two chief areas - the combat zone , or the area of active fighting , and the Communications Zone , or area required for administration of the theater . As the armies advanced , both these zones and the areas into which they were divided would shift forward to new geographic areas of control . History . British – American military staffs agreed during their meetings in Washington in January – March 1941 ( the ABC - 1 Conversations ) to exchange military missions to facilitate planning for the eventuality of American entry in the war . Major General James E . Chaney , an Army Air Corps officer , arrived in the United Kingdom on 18 May 1941 , and on the following day , Headquarters , Special Observer Group ( SPOBS ) , was established in London . SBOPS also had the role of studying British use of Lend Lease supplies . His formal title was the Special Army Observer in the United Kingdom and head of SPOBS . After the United States entered the war , SPOBS was succeeded by United States Army Forces in the British Isles ( USAFBI ) , actually SPOBS under a new name . At the time of the ARCADIA Conference , December 1941 - January 1942 , the decision was made to place the MAGNET forces ( U . S . Forces for Northern Ireland ) under the command of Maj . Gen . E . L . Daley , and make him in turn responsible to General Chaney , designated as CG , USAFBI . Lt . Gen . John C . H . Lee became Chief , Services of Supply , U . S . Army Forces British Isles , and later deputy theater commander , ETOUSA . On June 8 , 1942 , the United States Department of War officially established ETOUSA in its place . Its mission was to conduct planning for the eventual retaking of Europe and to exercise administrative and operational control over U . S . forces . The 133rd Infantry Regiment of the 34th Infantry Division was the first United States Army unit sent to Europe in World War II . The first battalion arrived in Belfast in late January 1942 , followed by the rest of the regiment in February . These units were designated as U . S . Army Northern Ireland Forces , later incorporated within the European Theater of Operations . The 133rd and 168th Infantry Regiments trained in the peat bogs , and performed border guard patrols between British Northern Ireland and the neutral Irish Free State . The remaining unit of the division , the 135th Infantry Regiment , arrived in May 1942 . From February 1944 the operational command was the Supreme Headquarters Allied Expeditionary Force ( SHAEF ) which as an Allied command also had operational control of British and all other allied land forces and tactical airforces in the European theatre . Until SHAEF was operational ETOUSA liaised closely with the British in the planning and organising of Operation Overlord . U . S . General Dwight D . Eisenhower had multiple command appointments ; he replaced Chaney in late June 1942 , but in November he also commanded the Allied forces in Operation Torch through AFHQ . He then relinquished command of ETOUSA to Lt . Gen . Frank M . Andrews in February 1943 , who was killed in an air crash in May . 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 Bernard Montgomery ( " " Monty " " ) , in the middle the American 12th Army Group commanded by General Omar N . 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 . The ETOUSA planning staff in London was usually referred to by its Army Post Office number , " APO 887 " . After the war in Europe ended , ETOUSA became briefly U . S . Armed Forces Europe , then U . S . Forces , European Theater ( USFET ) , and then , eventually , United States Army Europe . Campaigns and operations . Operation Torch — the invasion of French North Africa — involving the 9th , 3rd Infantry and the 2nd Armored Divisions , initiated on November 8 , 1942 , was the first ground combat operations for the United States in World War II . 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 " Europe first " 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 .