American logistics played a key role in the success of Operation Overlord, the Allied invasion of northwest Europe during World War II. The campaign officially commenced on D-Day, 6 June 1944, and ended on 24 July, the day before the launch of Operation Cobra. The Services of Supply (SOS) was formed under the command of Major General John C. H. Lee in May 1942 to provide logistical support to the European Theater of Operations, United States Army. From February 1944 on, the SOS was increasingly referred to as the Communications Zone (COMZ). Between May 1942 and May 1944, Operation Bolero, the buildup of American troops and supplies in the UK, proceeded fitfully and by June 1944 1,526,965 US troops were in the UK, of whom 459,511 were part of the COMZ.
The Overlord plan called for the early capture of Cherbourg, and a rapid American advance to secure the Brittany ports and Quiberon Bay, which was to be developed as a port. Crucially, the logistical plan called for a one-month pause at the Seine River, which Allied forces were expected to reach 90 days after D-Day, before advancing further. The expectation of an advance at a prescribed rate, while necessary for planning purposes, built inflexibility into a logistics plan that already had little margin for error. Staff studies confirmed that Overlord could be supported if everything went according to plan.
The First United States Army was supported over the Omaha and Utah Beaches, and through the Mulberry artificial port at Omaha specially constructed for the purpose, but the American Mulberry was abandoned after it was damaged by a storm on 19–22 June. During the first seven weeks after D-Day, the advance was much slower than the Operation Overlord plan had anticipated, and the lodgment area much smaller. The nature of the fighting in the Normandy bocage country created shortages of certain items, particularly artillery and mortar ammunition, and there were unexpectedly high rates of loss of bazookas, Browning automatic rifles (BARs), and M7 grenade launchers.