When the forces within a theater of operations consist of several field armies, a headquarters, known as the army group, is formed. An army group is the largest field organization handled by a single commander in modern warfare and usually includes between four hundred thousand and one-and-a-half-million troops. During WWII, only three army groups were formed: the 12th Army Group, commanded by General Omar Nelson Bradley, the Sixth Army Group, under General Jacob L. Devers; and the 15th Army Group, led by General Mark W. Clark.

The First US Group was activated 1943 in London, England.

The 6th Army Group. Their mission was to provide operational control over the French and American forces participating in the invasion of southern France.

The 12th Army Group was formed in 1944 from the former First United States Army Group. It was the largest command ever organized in US history.

The 15th Army Group was formed in Italy during WWII. This group had the responsibility of planning the invasion of Sicily.