Shipping lanes become a battlefield
The Battle of the Atlantic was a sustained Second World War campaign over maritime transport. Germany sought to disrupt Allied shipping, while Britain and its allies had to deliver merchant vessels, fuel and troops to their destinations. On 3 September 1939, the day Britain declared war, German submarine U-30 attacked the passenger liner Athenia. Civilian ocean travel faced direct danger from the outset.
The Admiralty quickly introduced convoys. Ocean-going groups began leaving Southend and Liverpool on 7 September, and convoy HX 1 sailed from Halifax, Canada, on 16 September. Early escorts did not cover entire passages; westbound ships might disperse after leaving certain dangerous coastal areas. Escort availability and a ship's ability to keep station directly affected attack risk.
The campaign expands to American coasts
Before formally entering the war, the United States had strengthened shipping control, creating a convoy and routing organization in November 1941. After entry, German submarines moved in force to US coastal waters. Existing patrol arrangements could not protect independently sailing merchantmen, while a shortage of escort vessels constrained convoys. Losses prompted naval authorities, sea-frontier commands and New York port officials to coordinate a new coastal network.
Regular Norfolk–Key West convoys began in mid-May 1942 and extended to Trinidad in early July, subsequently incorporating more Caribbean and Gulf of Mexico routes. They also had to connect with shipping between Boston and Halifax. The campaign brought previously dispersed port activities into interregional schedules and protection arrangements, subjecting tanker and dry-cargo operations to military coordination.
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The North Atlantic crisis in spring 1943
In early 1943, German submarine forces under Karl Dönitz concentrated on North Atlantic routes, using coordinated wolf packs in the mid-ocean gap beyond land-based aircraft coverage. Allied responses involved more than additional ships: they included long-range B-24 aircraft, wider escort-carrier air cover, and solutions to British, American and Canadian command and base-use issues.
In April, a US Army Air Forces antisubmarine detachment operated from St. John's, Newfoundland, using Royal Canadian Air Force joint command facilities. Both sides suffered losses in the battle around convoy ONS 5 in late April and early May. German forces subsequently withdrew from this phase of concentrated North Atlantic attack. Long-range aircraft, carrier aviation and radar together markedly improved the formerly dangerous routes by early summer.
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Connecting command, intelligence and technology
On 20 May 1943, Admiral Ernest King established the US Tenth Fleet to coordinate antisubmarine warfare in American areas of Atlantic responsibility. It had no directly assigned combat ships. Based in Washington, it connected operational direction, antisubmarine equipment and training, convoy routing and scientific advice. The Atlantic Fleet and sea-frontier commanders still conducted operations at sea, while intelligence and technical experience were exchanged centrally.
Convoy and routing work included checking merchant-ship speeds and equipment, holding masters' conferences, arranging sailing order and notifying commands of positions. Daily summaries exchanged with London and Ottawa recorded ships joining, straggling or changing course. Protecting transport depended not only on driving off submarines during battle but also on shore-based plotting, radio reports and international handovers of responsibility.
The campaign changes ports and trade organization
Wartime convoy schedules had to balance speed, escort numbers, season and congestion at destination ports. The US Navy's 1945 administrative history records that some slow convoys resumed departure from Halifax to relieve New York, while Norfolk became the main departure port in 1943 for some merchant ships bound for North Africa and the Mediterranean. Routing changed where cargo vessels assembled, waited and connected.
Sea transport also underpinned Allied military concentration in Europe. From August 1943 to April 1944, eleven UT troop convoys from New York to Liverpool carried approximately 592,000 military personnel. Fuel moved through a separate convoy system linking Curaçao, New York and Britain. Such flows show that convoy results ultimately meant people and supplies arriving, not merely numbers of submarines sunk.
Continued fighting after Allied superiority
Danger did not disappear once the Allies gained the advantage. German submarines remained active until May 1945. King's final report records US forces sinking a submarine near Block Island on 6 May; others surfaced to surrender only after the European war ended. The turning point in 1943 and the campaign's conclusion in 1945 were separate phases.
The campaign also produced denser Allied shipping coordination. Merchant mariners, escorts, long-range aviation and port authorities jointly sustained transoceanic supply. Shipbuilding replenished carrying capacity, while antisubmarine measures reduced risk; together they supported the landings in France and later ground offensives. Preserving maritime communications was necessary for those operations, but was not by itself the whole explanation for victory in Europe.