Tony Blair’s extraordinary decision to ask for Parliament’s approval for British military deployment in the Iraq War prompted lingering questions about who decides on matters of war and peace in modern Britain. His successors’ use, and thereby confirmation, of the new parliamentary prerogative suggested a fundamental reorganization of war powers in British politics, giving Parliament a significantly stronger position in the realm of foreign affairs. This paper argues that a number of factors, like a Prime Minister’s leadership style, the role Cabinet and the civil service, and Parliament’s governing disadvantages that makes it difficult for Members of Parliament to assert themselves proactively rather than reactively, make the prospect of a “War Powers Act” enshrining Parliament’s constitutional role in authorizing war highly unlikely.