The 'Battle of the St Lawrence' was a protracted but intermittent battle fought between German U-boats and largely Canadian naval and air forces in the lower reaches of the St Lawrence river and the whole of the Gulf of St Lawrence, Strait of Belle Isle, Anticosti island and Cabot Strait (May/October 1942, September 1943 and October/November 1944).

During this time, U-boats sank 23 merchant ships and four Canadian warships. There were several near-shore actions involving the delivery of German spies, or the attempted exfiltration of escaping prisoners of war. Despite the 23 ships lost, this battle marked a strategic victory for Canadian forces as ultimately they managed to disrupt U-boat activity, protect Canadian and Allied convoys, and intercept all attempted shore operations.

In the years between the two world wars, poor economic conditions and a sense of security, engendered by the proximity of the USA and the traditional protection of the Royal Navy, had resulted in the Royal Canadian Navy operation of only a very few ships, especially for coastal defence. Upgraded to six destroyers just before the war, Canadian naval deployment gave priority to the North Atlantic convoy routes. By the end of the war, the Royal Canadian Navy had expanded to become the third largest Allied naval power, with 400 vessels and 100,000 personnel. The Royal Navy contributed two destroyers to the fight in October 1942, the month in which the German attacks reached their peak.

From the outbreak of war in September 1939 until VE-Day in May 1945, several of Canada’s Atlantic coast ports became signally important to the resupply effort for the UK and later for the Allied land offensive in continental Europe. Halifax and Sydney, Nova Scotia, became the primary convoy assembly ports, with Halifax assigned the fast or priority convoys (largely troops and essential material) based on the more modern merchant ships, while Sydney was given responsibility for slow convoys which conveyed bulkier material on older and more vulnerable merchant ships. Both ports were heavily fortified with shore radar emplacements, searchlight batteries and extensive coastal artillery positions all manned by Royal Canadian Navy and Canadian army regular and reserve personnel. Military intelligence agents enforced strict black-outs throughout the areas and anti-torpedo nets were placed in harbour entrances. Even though no landings of German personnel took place near these ports, there were frequent attacks by U-boats on convoys departing for Europe. Less extensively used, but no less important, was the port of St John, which also saw the onward transfer of matériel, largely after the USA had entered the war in December 1941. The Canadian Pacific Railway’s main line from central Canada could be used to transport in aid of the war effort.

Although not crippling to the Canadian war effort, given the country’s rail network to the east coast ports, but possibly more destructive to the morale of the Canadian public, was the 'Battle of the St Lawrence' in which U-boats began to attack domestic coastal shipping along Canada’s eastern coast in the St Lawrence river and Gulf of St Lawrence from a time early in 1942 through to the end of the shipping season late in 1944.

The Kriegsmarine had made no formal plans to attack Allied merchant shipping in the St Lawrence river and the Gulf of St Lawrence, despite its activities off the convoy assembly ports of Halifax and Sydney, Nova Scotia, so early attacks in the 'Battle of the St Lawrence' were considered ad hoc and therefore opportunistic.

The first attack was by Korvettenkapitän Karl Thurmann’s U-553, which torpedoed and sank the 5,364-ton British freighter Nicoya at the mouth of the St Lawrence river off Anticosti island on 12 May 1942, followed by the 4,712-ton Dutch freighter Leto in the same vicinity several hours later. U-553 then left the Gulf of St Lawrence to return to its established patrol area in the North Atlantic.

Before these sinkings, the Gulf of St Lawrence and the St Lawrence river had been guarded by only four Canadian warships in the form if one 'Bangor' class minesweeper, two Fairmile Marine motor launches and one armed yacht, which was clearly a force inadequate for its task. The Royal Canadian Navy’s response to the attacks was to deploy into the area five 'Flower' class corvettes, but the defensive force remained inadequate even with these reinforcements.

The incident revealed that the Royal Canadian Navy lacked the resources to deal with the situation, and there were thus political repercussions in Canada with suggestions that Royal Canadian Navy warships allocated to the protection of Atlantic convoys be recalled to protect Canadian territorial waters, However, the Royal Canadian Navy’s priority remained the defence of convoys to the UK, the USSR and North Africa.

Several Royal Navy escort warships were attached to the Royal Canadian Navy for some months during 1942, with convoys in the St Lawrence river and the Gulf of St Lawrence being formed between Royal Canadian Navy base facilities at Chaleur II in Quebec City, Fort Ramsay in Gaspé and Protector in Sydney. Aircraft of the Royal Canadian Air Force flew operational patrols from air bases such as Mont-Joli, Bagotville, Chatham, Mount Pleasant, Charlottetown, Summerside, Debert, Stanley and Sydney as well as various civilian fields, particularly in the Magdalen islands group in the Gulf of St Lawrence.

Residents along the Gaspé coast, the St Lawrence river and the Gulf of St Lawrence were startled at the sight of maritime warfare off their shores, with ships on fire and explosions rattling their communities, while bodies and debris drifted ashore. The Canadian government’s wartime secrecy saw censors forbid media reporting of incidents. so the only news came from local gossip. Black-outs were strictly enforced and army units were sent out on coastal patrols along roads and railway lines.

In July 1942, Kapitänleutnant Ernst Vogelsang took his U-132 into the Gulf of St Lawrence. in less than 30 minutes on 6 July, he sank three ships of the 12-ship QS.15 convoy: these were the 2,555-ton British Dinaric, 4,312-ton Belgian Hainaut and 3,382-ton Greek Anastassios Pateras. Eventually depth charge runs by the minesweeper Drummondville and four Curtiss P-40 single-engined fighters of No. 130 Squadron damaged the boat’s ballast pumps and resulted in the loss of fuel and a few crewmen. This attack drove the boat to the bottom, where it hid for 12 hours. The boat then escaped from the Gulf of St Lawrence for repairs.

Late in August, two U-boats made a joint raid on the St Lawrence river. Kapitänleutnant Paul Hartwig’s U-517 sank nine ships and damaged another in a two-week period, escaping attacks by escort vessels each time and sinking the corvette Charlottetown on 11 September. Fregattenkapitän Eberhard Hoffmann’s U-165 was less successful in attacking merchant shipping but sank the armed yacht Raccoon and severely damaged the US oiler Laramie. The Royal Canadian Air Force’s Eastern Air Command positioned itself to offer better defence of the remaining convoys by establishing a 'Special Submarine Hunting Detachment' of No. 113 Squadron at Chatham, New Brunswick. This made its first attack on a U-boat on 9 September, when Pilot Officer R. S. Keetley dived on U-165 about 20 miles (32 km) to the south of Anticosti island. He did not do much damage to the U-boat, but subsequent naval and air activity in the area frustrated the U-boat’s efforts to attack other convoys. The Royal Canadian Navy requested additional forces from the Western Local Escort Force, receiving two old Royal Navy destroyers, with improved radar, with which to combat the U-boat threat.

Within 24 hours of 24 September, crews from No. 113 Squadron registered seven sightings and three attacks on Hartwig’s U-517. Flying Officer M. J. Bélanger made two of the attacks but did not sink the U-boat. Aircraft continued to harry the boat as it cruised in the Gulf of St Lawrence. Bélanger was in the cockpit for another attack on U-517 on 29 September, and though depth charges exploded all around the boat, it survived yet again. Still, Bélanger’s attacks had badly damaged the boat and wounded some of it men.

The continued U-boat attacks caused the St Lawrence river and the Gulf of St Lawrence to be closed to all trans-Atlantic shipping, allowing only coastal trade. In practice, although this embargo strained the Canadian National Railway line to Sydney and Halifax, it simplified the management of Atlantic convoys. The embargo lasted until early 1944.

In September 1942, Kapitänleutnant Heinz Walkerling’s U-91 and other boats attacked the ON.127 convoy and then pursued it across the Atlantic all the way to the gulf. The boat sustained minor damage from the escorting warships but managed to sink the ex-British destroyer Ottawa with two torpedoes.

In October, the 2,222-ton Newfoundland Railway passenger ferry Caribou was torpedoed by Kapitänleutnant Ulrich Gräf’s U-69 in the Cabot Strait between Sydney, Nova Scotia and Port aux Basques, Newfoundland, with heavy loss of life. The boat escaped a counterattack by the 'Bangor' class minesweeper Grandmère. In November, Kapitänleutnant Friedrich-Wilhelm Wissmann’s U-518 sank two iron ore freighters and damaged another at Bell Island in Conception Bay, Newfoundland, on its way to a patrol off the Gaspé peninsula where, despite an attack by a Canadian patrol aeroplane, it successfully landed a spy, Werner von Janowski. at New Carlisle, Quebec. The spy was captured at the New Carlisle railway station shortly after landing on the beach.

On 21 October U-43 moved into the entrance of the river and encountered widespread Canadian patrol activity. The boat’s captain, Oberleutnant Hans-Joachim Schwantke, attempted to attack the SQ.43 convoy off Gaspé, but was spotted and driven away by the convoy’s escorts. It was stated that six depth charges from the 'Bangor' minesweeper Gananoque knocked out the U-boat’s lights, blew the battery circuit breaker and activated a torpedo in one of the boat’s stern tubes. Schwantke pushed his boat to a depth of almost 430 ft (130 m) to avoid what he thought was a co-ordinated attack, and while it was damaged it managed to escape from the river.

In November, Kapitänleutnant Heinrich Schäfer’s U-183 was ordered into the Gulf of St Lawrence but turned away in the face of many Canadian patrols.

U-boat losses during 1942 following the entry of the US Navy into the 'Battle of the Atlantic', together with declining German shipbuilding capability to replace battle losses, saw the U-boat fleet redeployed to the primary Atlantic convoy routes designed to disrupt the Allied war resupply effort, and this effectively saw the withdrawal of the U-boats from the St Lawrence river and the Gulf of St Lawrence by the end of 1942.

Early in 1943, Canadian military intelligence and the Royal Canadian Mounted Police intercepted mail addressed to several Kriegsmarine officers, including the U-boat 'ace' Fregattenkapitän Otto Kretschmer, held at the Camp 30 prisoner of war camp at Bowmanville, Ontario. The correspondence detailed the 'Kiebitz' escape plan in which the prisoners were to tunnel out of the camp and make their way (using currency and false documents provided for them) through eastern Ontario and across Quebec to the north-eastern tip of New Brunswick off the Pointe de Maisonnette lighthouse, where they would be collected by a U-boat. The Canadian authorities did not tip off the prisoners and detected signs of tunnel digging at Camp 30. All prisoners except one were arrested at the time of their escape attempt. The one man who did manage to escape travelled all the way to Pointe de Maisonette undetected, probably travelling on the Canadian National Railway passenger trains to the Bathurst area. The man was apprehended by military police and Royal Canadian Mounted Police on the beach in front of the lighthouse on the night of the planned U-boat extraction.

The Royal Canadian Navy provided the 'Pointe Maisonnette' U-boat counter-offensive force led by the 'Flower' class corvette Rimouski, which was outfitted with an experimental version of diffused lighting camouflage for the operation. Rimouski's task force waited in Caraquet harbour, obscured by Caraquet island, during the night of 26/27 September and detected the presence of Kapitänleutnant Rolf Schauenburg’s U-536 off Pointe de Maisonnette while shore authorities arrested the escapee. U-536 managed to elude the Canadian task force by diving just as the surface warships began attacking with depth charges, and was able to escape the Gulf of St Lawrence, although without making the planned extraction.

In 1943, the Royal Canadian Air Force had begun a successfully programme to harass U-boat operations in Canadian coastal waters and the Royal Canadian Navy had grown sufficiently in numbers and capability to allow the dedication of greater resources to anti-submarine warfare operations in territorial waters. By a time early in 1944, the shipping lanes in the Gulf of St Lawrence and the St Lawrence river were reopened to domestic and war-related convoys operating primarily from the city of Quebec to Sydney.

Late 1944 saw a resurgence of U-boat activity in the St Lawrence river and Gulf of St Lawrence. Many of the German boats were now equipped with the Schnorchel, a telescopic engine ventilation system that permitted continuous underwater operation.

Oberleutnant Albert Knelp’s U-1223 entered the Gulf of St Lawrence without being detected early in October and is credited with seriously damaging the 'River' class frigate Magog on 14 October and sinking the Canadian freighter Fort Thompson on 2 November. Three weeks later, Oberleutnant Friedrich-Wilhelm Marienfeld’s U-1228 attacked and sank the 'Flower' corvette Shawinigan a short distance off of Channel/Port aux Basque on the night of 24/25 November, with the loss of all 91 crew members. The authorities realised that the corvette had been sunk only when Caribou's replacement ferry, Burgeo, sailed into North Sydney without Shawinigan on 26 November after unsuccessfully attempting to make radio contact earlier in that same day. Wreckage was discovered on 27 November, and six crewmen’s bodies were recovered.

This pair of German attacks marked the end of the 'Battle of the St Lawrence'. In May 1945, following Germany’s surrender, U-889 and U-190 reached the Royal Canadian Navy bases at Shelburne, Nova Scotia and Bay Bulls, Newfoundland respectively.

After the war, it was shown that the mingling of fresh and salt waters in the region, which is the world’s largest estuary, combined with temperature variations and sea ice to disrupt disrupted Canadian anti-submarine operations and reduced the effectiveness of shipboard sonar systems. Fog and other weather conditions in the St Lawrence river and Gulf of St Lawrence also adversely affected Royal Canadian Air Force patrols.