Along with air power, the BdU was forced to contend with a large increase in available Allied convoy escorts which replenished their tanks from tankers in the convoys allowing escort across the ocean. The escort carrier support groups, protected by destroyers, which, in the words of the official naval historian of the Second World War, proved decisive; "it was the advent of the Support Groups, the Escort Carriers and the Very Long Range Aircraft which turned the tables on the U-boats-and did so with astonishing rapidity."