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.

Background

During the 1920s and 1930s, the United States had developed and periodically updated War Plan Black for the possibility of a war with Germany. Planning began in earnest at the ABC-1 Conference in Washington, D.C., in January to March 1941, where agreement was reached with the UK and Canada on a Europe first strategy in the event of the US being forced into a war with both Germany and Japan. A US military mission to the UK called the Special Observer Group (SPOBS) was formed under the command of Major General James E. Chaney, a United States Army Air Corps officer who had been stationed in the UK since October 1940 to observe air operations. Chaney opened SPOBS headquarters at the US Embassy at 1 Grosvenor Square, London, on 19 May 1941, and moved it across the street to 20 Grosvenor Square two days later. Henceforth, Grosvenor Square would be the hub of American activity in the UK.

In the wake of the American entry into World War II in December 1941, the War Department activated the United States Army Forces in the British Isles (USAFBI) under Chaney's command on 8 January 1942. Under the ABC-1 and Rainbow 5 war plans, the United States would participate in the defense of the UK, but the Chief of Staff of the United States Army, General George C. Marshall, had a larger role in mind. In April 1942, Marshall and Harry L. Hopkins, the principal foreign policy advisor to the president, visited the UK, and obtained the approval of the prime minister of the United Kingdom and the British Chiefs of Staff Committee for Operation Bolero, the buildup of US forces in the UK with the aim of eventually mounting a cross-Channel attack. With this prospect in mind, Chaney asked the War Department for personnel to form a Services of Supply (SOS) command.

Chaney's proposed SOS organizational structure did not meet with the approval of the War Department. On 9 March 1942, Marshall had conducted a sweeping reorganization that consolidated logistical functions in the US under the United States Army Services of Supply (USASOS), headed by Major General Brehon B. Somervell. Chaos had resulted during World War I because the organization of the SOS in France was different from that of the War Department, and an important lesson of that war was the need for the theater SOS organization to parallel that in the United States. Marshall and Somervell wanted it led by someone familiar with the new organization, and selected Major General John C. H. Lee, who had previously commanded the Pacific Ports of Embarkation in the United States. Each branch head in Somervell's headquarters was asked to nominate his best two men, one of whom was selected by Somervell and Lee for SOS headquarters, while the other remained in Washington. Lee held the first meeting of his new staff on 16 May, before departing for the UK on 23 May, and Chaney formally activated SOS the following day.

On 8 June 1942, the War Department upgraded USAFBI to the status of a theater of war, becoming the European Theater of Operations, United States Army (ETOUSA). Chaney was recalled to the US, and replaced by the chief of the Operations Division at the War Department, Major General Dwight D. Eisenhower. Accommodation for the SOS headquarters was initially provided in an apartment building at No 1 Great Cumberland Place in London, but something larger was required. Brigadier General Claude N. Thiele, the SOS Chief of Administrative Services, found of office space in Cheltenham. Intended as an evacuation point for the British War Office in the event that London had to be evacuated, the staff stationed there were in the process of moving back to London. The site had adequate road, rail and signal communications, but the distance from London was a disadvantage. Lee opened SOS headquarters in Cheltenham on 25 July.

Lee announced a regional organization of SOS on 20 July. It was divided into four base commands corresponding to the British Army's territorial commands. The Northern Ireland Base Section, under Brigadier General Leroy P. Collins, with its headquarters at Belfast, included all of Northern Ireland; the Western Base Section, under Davison, had its headquarters at Chester; the Eastern Base Section, under Colonel Cecil R. Moore, had its headquarters at Watford; and the Southern Base Section, under Colonel Charles O. Thrasher, had its headquarters at Wilton, Wiltshire. The London Base Command, created within ETOUSA in 1942 under the command of Brigadier General Pleas B. Rogers, was transferred to SOS on 21 March 1943 and renamed "Central Base Section" on 29 April. Over time each of the base sections acquired its own character, with the Western Base Section principally concerned with the reception of troops and supplies, the Eastern with supporting the Air Force, and the Southern with hosting marshalling and training areas. The doctrinal concept behind the base section concept was of "centralized control and decentralized operation", but reconciling the two proved difficult in practice.

Planning and preparations Bolero

Bolero was derailed by the decision taken in July 1942 to abandon Operation Sledgehammer, the proposed 1942 cross-channel attack, in favor of Operation Torch, an invasion of French Northwest Africa. This made Operation Roundup, the prospective cross-channel attack in 1943, unlikely. Doubtful that sufficient shipping was available to support both Bolero and Torch, Somervell ordered all construction work in the UK to cease, except on airfields, but the British government went ahead anyway, using materials and labor supplied under Reverse Lend-Lease. After the Casablanca Conference in January 1943, American resources became available again, and eventually the works completed included of covered storage, of open storage and hardstands, and facilities for storage of of petrol, oil and lubricants (POL).

Roundup was not carried out, but Bolero survived, strengthened by the decision at the 1943 Washington Conference (codenamed "Trident") to mount the cross-channel attack with a target date of 1 May 1944. The planners at Trident envisioned shipping 1.3 million US troops to the UK by that date. Effecting this required the ports in the UK to handle up to 150 ships per month. Progress was disappointing, but record shipments of men in the last quarter of the year boosted the strength of ETOUSA to 773,753 by the end of 1943, of whom 220,200 were in the SOS.

To take advantage of the longer daylight hours of summer, a system of preshipment was instituted, whereby unit equipment was shipped to the UK in advance of the units. Rather than lose training time packing all its equipment, personnel could sail to the UK, and draw a new set of equipment there. The major obstacle to the idea was that not all commodities were available in surplus in the US. Indeed the Army Service Forces (ASF), as USASOS had been renamed on 12 March 1943, had difficulty filling the available shipping. Of of cargo space available in July, only were used; of available in August, only . Of the shipped in May through August, 39 percent was preshipped cargo. This rose to or 54 percent of the shipped in November, but most of this was consumed re-equipping three of the four divisions that were transferred from the North African Theater of Operations (NATOUSA).

The main points of entry for US cargo were the ports on the Clyde and the Mersey, and those of the Bristol Channel; ports on the south and east coasts of the UK were subject to attack by German aircraft and submarines, and were avoided until late 1943, when shipments began to exceed the capacity of the other ports. The Clyde ports were remote from the main supply depots, but were used as the main debarkation points for US troops, accounting for 873,163 (52 percent) of the 1,671,010 US personnel arrivals, but only (8 percent) of cargo.

The majority of troops travelled across the Atlantic on ocean liners like the and . Making three round-trip voyages per month carrying up to 15,000 passengers each time, these two liners alone carried 24 percent of troop arrivals. They were supplemented by other liners, including the and , and the , and , which accounted for another 36 percent. The troops debarked into ship's tenders, and boarded quayside trains to their destinations.

The Bristol Channel ports and the Mersey ports handled (70 percent) of the cargo brought to the UK, including most of the heavy items like tanks, artillery pieces and ammunition. This was not accomplished without difficulty; most of the cargo handling equipment was old and outdated, and it was not possible to follow the standard US practice of moving goods from the quayside on pallets with forklifts. Trade unions opposed the use of military labor except when civilian labor was unavailable, but this ban was lifted when the volume of cargo became too great, and by May 1944 fifteen US port battalions were working the UK ports.

SOS found the textbook concept of shipping cargo from the port to distribution centers, sorting it there, and despatching it to branch depots too extravagant in its use of scarce depot space and the overburdened British railway system. Moreover shipping manifests were often found to be incomplete, inaccurate or illegible, and despite being sent by air mail still frequently failed to arrive in advance of the cargo. SOS ultimately persuaded a reluctant ASF to accept a system whereby every item shipped was individually labelled with a requisition number that provided a complete UK destination address.