Throughout
World War II,
Britain and the
European theatre of war in general was heavily reliant on supplies from the
USA, the
Empire and elsewhere, which had to be brought in by sea. Initially the Germans' attempts to disrupt these trade routes involved a mixture of
submarine (
U-boat) attacks and surface raiders (battleships, heavy cruisers and disguised merchantmen), but the limited success of the surface fleet even after its choice of bases was expanded by the capture of
Norway and
France in 1940 fairly rapidly led to concentration on a U-boat based strategy.
The British response was to set up a system of convoys (as had been done during the latter years of World War I) escorted by units with anti-submarine capabilities, initially destroyers - particularly older classes and the fifty old flush-deckers supplied by the USA on Lend-Lease - and converted trawlers, but later on cheaper specialist units such as corvettes (the Flower class), frigates (the River class) and escort destroyers (the Hunt class). From the American entry into the war - or indeed slightly before - US Navy destroyers also played their part.
The balance ebbed and flowed with improvements in technology and tactical doctrine on both sides - for the allies improved radar and asdic for detection, anti-submarine weapons like the Hedgehog and Squid depth charge projectors, and the breaking of the Germans' ciphers by Ultra, while the Germans developed wolfpack tactics, the snorkel and finally closed cycle H2O2 engines, but by 1943 the Germans seemed to be gaining the upper hand, with unsustainable losses to the allies; however increased air cover as longer range planes and escort carriers became available, as well as the production capacities of American shipyards and further gains in the cryptographic war, finally swung the advantage towards the allies.
Casualties on both sides were severe. By the end of the war, some 85% of German U-boat crews had been killed or (more rarely) captured. 30 000 seamen from the British merchant navy (which employed crews from all parts of the world, especially the parts with low rates of pay) and 7000 on ships flying the American flag lost their lives; for the Americans it was a higher casualty ratio than that of any of the combatant forces except the US Marines. As non-combatants, the seamen were not granted the rights of those in the armed forces - in British service a seaman whose ship was sunk had no right to pay after the day of the sinking, and there was no paid leave. Their widows were denied the additional benefits paid to widows of serving members of the armed forces, and their names rarely appeared on war memorials (although there is a memorial to them on Tower Hill, London). The USA finally recognised its merchant seamen as war veterans in 1988; the UK has yet to do so.
Chronology to follow when time permits and my books are no longer all in boxes.
Sources (apart from, like, stuff I know):
http://home.onet.co.uk/~canonesa
http://www.lascars.co.uk/war.html
http://www.infoplease.com/ipa/A0872853.html