Declarations of war by Great Britain and the United Kingdom
A declaration of war is a formal declaration issued by a national government indicating that a state of war exists between that nation and another.
Background
In the United Kingdom, the government and command of the armed forces is vested in the sovereign. Under the sovereign, direct control of the armed forces is divided between the government and the Defence Council. However, a constitutional convention has developed regarding parliamentary approval for military action.There has been a long-running debate regarding whether Parliament alone should have the power to declare war and more widely to commit British forces to armed conflict. This was attempted in 1999 with the introduction of the Military Action Against Iraq Bill. However Queen Elizabeth II, acting upon the advice of her government at the time, refused to grant her consent to allow the bill to be debated in Parliament and so it was dropped. The Constitutional Reform and Governance Act 2010 originally included a section that would have required Parliamentary approval for use of the armed forces, but this was dropped from the bill before royal assent.
There have been no declarations of war since the Second World War, though British Armed Forces have taken part in armed conflict on numerous occasions nonetheless. A committee of the House of Lords opined that the prohibition on initiating violent conflict in the Charter of the United Nations has made declaration of war redundant as a formal international legal instrument.
The procedure for a declaration of war is set out in a letter dated 23 August 1939 from Gerald Fitzmaurice. It reads: