Britain depended on vital supplies from North America and the Empire in the Second World War. These had to be transported in merchant ships across the Atlantic Ocean, where they could be attacked by German submarines (U-boats) and warships.
To combat this threat, the merchant ships were grouped into convoys escorted by warships and, if possible, aircraft. The first Atlantic convoy sailed on 2 September 1939.
At first, many merchant ships were lost. The fall of France in June 1940 gave the U-boats bases on the Atlantic coast, and U-boat production increased during spring 1941, giving the Germans enough submarines to deploy groups or 'wolf packs'.
Winston Churchill coined the phrase 'Battle of the Atlantic' on 6 March 1941, deliberately echoing the Battle of Britain to emphasise its importance.
In May 1941, the loss of the German battleship Bismarck ended surface raids, and the Allies extended the convoy system right across the Atlantic. Intelligence successes allowed the Allies to route convoys away from danger, and losses finally fell.
The German battleship BISMARCK firing at HMS HOOD during the action in the Denmark Strait. This photograph was taken from the German heavy cruiser PRINZ EUGEN.
After the United States entered the war in December 1941, the Germans sank nearly 500 un-escorted ships off the US east coast in early 1942 until the Americans introduced convoys.
A convoy with escorting battleship
The Atlantic covers over 100 million square kilometres. In such a vast space, finding a convoy was harder than finding a single ship, as there were fewer targets, and attacking U-boats could be attacked by the escorting warships. Convoys were formed into several columns of ships, with up to five ships in each column, forming a big box of up to 60 ships.
The wolf packs returned to the mid-Atlantic. A temporary Allied inability to read their signals meant that by the end of 1942, Allied shipping was in crisis. The introduction of aircraft carriers, Very Long Range aircraft and roving 'support groups' of warships eventually defeated the U-boats at the end of May 1943.
This article was edited by Amy Carmichael. Several IWM staff members contributed to an older version of this piece.