American logistics in the Normandy campaign 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.