3 September Britain and France declare war on Germany. The U-boat U-30 sinks the British liner Athenia without warning off the Scottish coast; 112 passengers die.

17 September The Royal Navy carrier Courageous is sunk off Ireland by a U-boat.

14 October Battleship HMS Royal Oak sunk in Scapa Flow harbour in Orkney by U-47 commanded by Gunther Prien.

1940

June The fall of France gives U-boats Atlantic bases on the Bay of Biscay with a command centre at Lorient. Mainstay of the Atlantic U-boat fleet is the Type VII 'Sea Wolf'.

July-October The first 'happy time' for the U-boats, during which they sink 217 merchant ships, most of them unescorted. The first U-boat 'aces' emerge, among them Otto Kretschmer and Wolfgang Luth.

1941

7 March Gunther Prien is killed when the British destroyer Wolverine sinks U-47.

17 March HMS Vanoe uses Type 286 radar for the first time to detect and sink U-100.

9 May The crew of a Royal Navy escort capture U-110 and its Enigma enciphering machine and codebook.

17 December The Royal Navy's first escort aircraft carrier, HMS Audacity, covering Convoy HG76, secures its first kill, U-131.

21 December After a running battle, Audacity is sunk by U-751.

1942

January The second U-boat 'happy time' begins as five Type XI U-boats arrive off the American eastern seaboard and play havoc with unescorted shipping.

2 January US Atlantic Fleet airships ('blimps') begin to make flights to protect Allied shipping. By the end of the war, they will have made 37,554 flights and escorted over 70,000 vessels.

February The U-boats move south to the waters off Miami.

April Introduction of first 'milch cow' supply U-boat: U-459.

May 41 Allied ships sunk by U-boats in the Gulf of Mexico.

July A new system of interlocking Allied convoys brings the second 'happy time' to an end.

1943

16 March Three U-boat wolf packs, totalling 37 U-boats, attack Allied convoys SC122 and HX229 in the mid-Atlantic 'air gap' (where Allied air coverage does not extend). A running battle ensues in which 21 ships are sunk. By the end of March, 120 Allied ships have been sunk. The Battle of the Atlantic hangs in the balance.

April RAF Coastal Command supplements its Sunderland and Catalina anti-submarine flying boats with very long-range (VLR) bombers flying from Iceland, Northern Ireland and Newfoundland, carrying the Mark 24 mine. Three new escort carrier groups are introduced plus merchant aircraft carriers (MACs) .

28 April The heavily escorted convoy ONS5, sailing west from Britain, is engaged off Newfoundland by more than 30 U-boats. The battle lasts until 6 May. The escorts sink six U-boats and badly damage another five. The U-boats sink only five ships.

May Doenitz loses 43 U-boats and calls his wolf packs home.

30 July Allied patrol aircraft surprise two milch cows and U-504. They sink U-462, one of the milch cows, as well as U-504.

September 'Squid' mortars, capable of firing depth charges in front of escorts and coupled with advanced ASDIC equipment, are fitted to Allied escorts.

October 23 U-boats are sunk in attacks on convoys. They are forced to spend more time submerged and rely on German patrol aircraft to find their prey.

1944

January Allied carriers and shore-based aircraft now provide full air cover to every convoy.

19 February U-264, the first U-boat fitted with a schnorkel, is sunk.

June Doenitz loses 11 U-boats in attacks on ships supporting the Allied invasion force in Normandy.

31 July The sloop HMS Sterling makes the first 'Squid' kill, sinking U-333 in the Channel.

August Doenitz loses his U-boat bases on the Bay of Biscay as Allies overrun France.

1945

February Type XXIII U-boats begin operational patrols off the east coast of Britain, sinking six ships with no losses themselves.

25 April First Type XXI U-boat goes operational but is able to sink no ships before being ordered to surrender by Doenitz a week later.