Convoys faced a full spectrum of threats

Early-war Axis forces threatened convoys with surface raiders, wolf packs, independently operating submarines, mines, and aircraft. The attack inside Scapa Flow shows what a lone submarine could attempt, while other boats operated independently to lay mines. Surface raiders affected convoy tactics through their presence and the fear they created even when they did not attack. Each additional type of threat complicated escort formations and responses. Once the Axis campaign narrowed mainly to submarines, escorts could concentrate their effort and the Allied problem became more manageable.