[3] Upgraded to six destroyers just before the war, Canadian naval deployment gave priority to the North Atlantic convoy routes.

From the start of the war in 1939 until VE Day, several of Canada's Atlantic coast ports became important to the resupply effort for the United Kingdom and later for the Allied land offensive on the Western Front.

Both ports were heavily fortified with shore radar emplacements, searchlight batteries, and extensive coastal artillery stations all manned by RCN and Canadian Army regular and reserve personnel.

Military intelligence agents enforced strict blackouts throughout the areas and anti-torpedo nets were in place at the harbour entrances.

The Canadian Pacific Railway mainline from central Canada (which crossed the state of Maine) could be used to transport in aid of the war effort.

The Canadian government's wartime secrecy saw censors forbid media reporting of incidents; so the only news came from local gossip.

On 6 July, within half an hour, he sank three ships from the twelve-ship convoy QS-15: the British registered Dinaric and Hainaut, and the Greek vessel Anastassios Pateras.

U-517 sank nine ships and damaged another in a two-week period, escaping attacks by escort vessels each time and sinking the Flower-class corvette HMCS Charlottetown on 11 September.

U-165 was less successful in attacking merchant shipping but it sank the armed yacht HMCS Raccoon and heavily damaged USS Laramie.

In practice, although this embargo strained the Canadian National Railway (CNR) system to Sydney and Halifax, it simplified the management of Atlantic convoys.

that six depth charges from the Bangor-class minesweeper HMCS Gananoque knocked out the U-boat's lights, blew the battery circuit breaker and activated a torpedo in one of the sub's stern tubes.

On 2 November, U-518 sank two iron ore freighters and damaged another at Bell Island in Conception Bay, Newfoundland, en route to a patrol off the Gaspé Peninsula where, despite an attack by an RCAF patrol aircraft, it successfully landed a spy, Werner von Janowski at New Carlisle, Quebec; he was captured at the New Carlisle railway station shortly after landing on the beach.

U-boat losses experienced by the Kriegsmarine during 1942 following the entry of the United States Navy into the Battle of the Atlantic, coupled with declining German shipbuilding capability to replace battle losses, saw the U-boat fleet redeployed to the primary Atlantic convoy routes to disrupt the Allied war resupply effort; this effectively saw enemy submarines withdrawn from the St. Lawrence River and Gulf of St. Lawrence by the end of 1942.

The correspondence detailed an 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 northeastern tip of New Brunswick off the Pointe de Maisonnette lighthouse where the escapees would be retrieved by a U-boat.

U-536 managed to elude the RCN task force by diving just as the surface warships began attacking with depth charges; the submarine was able to escape the Gulf of St. Lawrence without making the extraction.

German submarines were being equipped with the snorkel, a telescopic engine ventilation system that permitted continuous underwater operation without surfacing.

[9] Three weeks later, U-1228 attacked and sank the Flower-class corvette HMCS Shawinigan, a few kilometres off of Channel/Port aux Basque on the night of 24–25 November, with the loss of all 90 crew members, including able seaman Dudley "Red" Garrett, a former Toronto Maple Leafs hockey player.

[10] Authorities only realized that it sank when Caribou's replacement ferry, SS Burgeo, sailed into North Sydney without Shawinigan on 25 November, after it had tried numerous times to make contact by radiophone earlier that day.

After the war, it was shown that the mingling of fresh and salt waters in the region (the world's largest estuary), plus temperature variations and sea ice, disrupted RCN anti-submarine operations and reduced the effectiveness of shipboard sonar systems that were designed to detect submarines.