In March 1943 the Germans come close to overwhelming even well-escorted convoys. Between the 7th and 11th, slow convoy SC121 loses 13 ships. Then, in the largest convoy battles of the war, between March 16th and 20th over 40 U-boats are deployed against convoys HX229 and SC122. They sink 21 merchantmen before additional air and surface escorts finally drive them off. Only one U-boat is sunk. German victory in the Atlantic seems near- and with it the collapse of any plans for an Allied invasion of Western Europe.

However, the naval war is about to turn dramatically in the Allies' favour due to a series of developments:

1. The Allies' code breakers finally break the new German naval code, known as "Triton". The Allies can read the secret communications between the U-boats and the German naval command and know where and when U-Boats will rendezvous with supply ships, or where they are waiting to ambush a convoy.

2. Use of much improved High Frequency ship-borne radio direction finders, allow warships to home in on U-Boats by their radio transmissions. The new surface level 10cm wavelength radar to allow ships to detect U-Boats on the surface, even at night or in poor visibility.

3. The Allies Created special escort and hunter squadrons of ships using team tactics created by Captain Frederick Walker, the most successful RN U-Boat hunter. The first five Royal Navy support groups with modern radars, anti-submarine weapons and escort carriers are put into full operation in the North Atlantic.

3. Anti-submarine aircraft are being fitted with the 10cm wavelength radar which is undetectable by the U-boats. As more VLR ( very long range) Catalina, Canso, B-24 Liberator and Short Sunderland long range aircraft join Coastal Command they strengthen the Allies control of the convoy routes. The aircraft can spot U-Boats directly from the air, or use surface radar, or even identifying them up to 100 feet underwater.

Here is a WW2 film clip of an Allied aircraft attacking a U-Boat:

RAF Sinks U-Boat off Ireland 1:05