Forbes served in the Second World War as Commander-in-Chief Home Fleet, in which role he transferred his flag to the battleship in December 1939. His fleet suffered heavy losses including the aircraft carrier and nine destroyers during the Norwegian campaign in Spring 1940. He was on board "Rodney" when she came under air attack and was hit by a 500 kg (1,103 lb) bomb that pierced the armoured deck on 9 April 1940. Promoted to Admiral of the Fleet on 8 May 1940 and advanced to Knight Grand Cross of the Order of the Bath on 11 July 1940, he became Commander-in-Chief, Plymouth in May 1941. In that capacity he organised the defence of Plymouth from air attack, prosecuted attacks on enemy shipping using the harbour at Brest as well as other ports along the French coast, and also initiated the St Nazaire Raid in March 1942.