The Statute of Westminster
Upon request from the British Dominions, and with their consent, the British Parliament adopted the Statute of Westminster on December 11, 1931. The Statute gave to the former British colonies of Canada, Newfoundland, Australia, New Zealand and South Africa as well as the Irish Free State full judiciary powers, except when and where a dominion states that it will remain under British legislation. By allowing them to enact legislation that can come into effect outside their territories, the Statute of Westminster authorizes the parliaments of the British Dominions to adopt and implement their own foreign policy, independently from London. That clause includes the right to declare war on another country.