'Accumulator' was a British diversionary naval undertaking off the German-occupied Channel Islands and the coast of northern France, simulating an amphibious assault on the area of Coutances and Granville, in support of 'Overlord' by holding German formations on the western side of the Cotentin peninsula (12/13 June 1944).
As part of 'Fortitude', a series of deception operations had been used to divert German attention from the Allied landings by suggesting that a second invasion force was still waiting to land. This caused the defenders to divert their forces from the fighting in Normandy, holding them in reserve for an invasion in the Pas de Calais, considerably farther to the north-east.
Some days after the initial 'Overlord' landings, it was decided to mount a small-scale operation to simulate a follow-up landing force heading for Coutances and Granville, on the western side of the Cotentin peninsula, and thereby persuade the German command organisation headed by Generalfeldmarschall Gerd von Rundstedt, the Oberbefehlshaber 'West', either to pull units from the front line and redeploy them to protect the western coast, or to hold units already there and not redeploy them to the main front.
Two Canadian destroyers, Haida and Huron, were therefore despatched to make a series of spurious radio transmissions for the Germans to intercept. The beginning of the operation went smoothly as the two ships signalling that the invasion fleet had been delayed by engine problems, and giving a revised plan. However, Haida's radios broke down, forcing Huron to continue alone. The two ships were also detected by an Allied reconnaissance aeroplane, which radioed the information that it had found 'unidentified warships'.
The operation was thus a failure and, while the intended signals had been made, there was no reaction from the German forces.