Following the signing of the Treaty of Versailles in 1919, Germany was not permitted to build or operate submarines. Clandestine training about Finnish and Spanish submarines took place and U-boats were still built to German designs in Dutch yards.

At the outset of the Second World War, Dönitz argued for a 300-strong U-boat fleet, since his force of 57 U-boats 'could only inflict pin-pricks against British seaborne trade'. In August 1939, U-48 left Germany, commanded by 'Vaddi' Schultze, to take up a waiting position around England.

It scored its first success on 5 September, when it torpedoed the British freighter Royal Sceptre, then the Winkleigh on 8 September. On both occasions – the first of many – Schultze showed himself to be a notable humanitarian: he addressed signals to Churchill giving positions of the sinkings so that crews could be saved.

By 1 August 1941, U-48, the most successful boat of the Second World War, had sunk 56 merchant ships of 322,478 gross tons and one corvette. She was then transferred to the Baltic as a training boat. Schultze became commander of operation 3 U-Flotilla and later was appointed commander, II/Naval College Schleswig. He died in 1987 at the age of 78. U-48 was scuttled on 3 May 1945.