Why Parliament Now Decides on War: Tracing the Growth of the Parliamentary Prerogative through Syria, Libya and Iraq

• Precedents set in debates over Iraq, Libya and Syria established a new parliamentary prerogative, that MPs must vote before military action can legitimately be launched.
• Tony Blair conceded the Iraq vote to shore up Labour back-bench support, because he was convinced he would win, and because he was unwilling to change course regardless.
• David Cameron allowed a vote on Libya because he believed parliament should have a say, because UN support meant he was certain to win, and to gain plaudits for not being Blair.
• Cameron then had to allow a vote on Syria despite its greater political sensitivity. He mishandled the vote, and lost, and felt constrained to pull out of mooted military action.
• Collectively these three precedents comprise a new constitutional convention, which will constrain the executive in future whether the law is formally changed or not.

Parliament now decides when Britain goes to war. The vote against military intervention in Syria on 29 August 2013 upheld a new parliamentary prerogative that gradually developed through debates over earlier actions in Iraq and Libya. While the academic community and much of the British political elite continue to focus on the free rein granted to prime ministers by the historic royal prerogative, this article argues it is critically constrained by its parliamentary counterpart. It traces the way political conditions, individual policymaker preferences, and the conventional nature of the unwritten British constitution allowed parliament to insert itself into the policymaking process without the consent of successive governments. It concludes that MPs will in future expect the right to vote on proposals to deploy the armed forces overseas, and that the legitimacy of military action will depend on the government winning such a vote.