They were the subject of a major U-boat attack in September 1943, the first battle in the Kriegsmarine's autumn offensive, following the withdrawal from the North Atlantic route after Black May.

Following the defeats of May 1943, and the devastating losses incurred by the U-boat Arm (U-Bootwaffe, or UBW) Admiral Dönitz had withdrawn from attacks on the North Atlantic route while awaiting tactical and technical improvements.

Chief among these was the T-5 acoustic torpedo, with which (it was planned) the convoy escorts could be attacked and eliminated, leaving the merchant ships defenceless.

By September 1943 these were ready, and U-boat Control (Befehlshaber der Unterseeboote, BdU) dispatched a patrol group of 21 boats, code-named Leuthen, to renew the attack on the North Atlantic route.

ON 202 had left Liverpool on 15 September, composed of 38 ships and escorted by Canadian escort group C-2, comprising 2 destroyers, Gatineau (commanded by Lt.Cdr PW Burnett RN, SOE) and Icarus; the frigate Lagan, and 3 corvettes; Drumheller, Kamloops and Polyanthus.

Support Group 9 comprised destroyer St Croix, frigate Itchen (Cdr CE Bridgman RN, SOE) and 3 corvettes, Chambly, Morden and Sackville.

U-338 was attacked by a Liberator of 120 Sqdn RAF which was credited with the kill; however post-war analysis showed that U-338 was only damaged by this, though was later engaged and sunk by the corvette Drumheller of C 2 group.

U-238 fired on 2 ships, sinking one, Theodore Dwight Weld, and damaging another, Frederick Douglass.

On 23 September the convoys reached the Grand Banks area, where fog hindered visibility both of the air patrols and the attacking Leuthen boats.