The decision was made well before the war’s outbreak. In case of Japanese invasion, Great Britain did not expect to be able to prevent the conquest of its prized Chinese Crown colony—Hong Kong. Instead, Prime Minister Winston Churchill had ordered Major General Christopher Maltby to delay the inevitable for as long as possible, fighting a protracted battle from the mainland onto Hong Kong Island before surrendering. British resistance in indefensible Hong Kong would hopefully buy more time for the troops defending Malaya and Singapore, which was Churchill’s main strategic focus.
In accordance with that plan, since the outbreak of war in Europe the Royal Navy had been evacuating all blue-water warships from Hong Kong to the naval base at Singapore to prevent their capture by the Japanese. The 12 vessels of the 5th Cruiser Squadron had all departed by the end of 1940, followed by the nine boats of the 4th Submarine Flotilla.