Engaging democratically elected assemblies in national decision-making over the extraterritorial use of force seemingly provides a secure check on executive abuses of power. Many liberal democracies therefore maintain constitutional requirements that their elected national assembly must authorize decisions to use military force. By comparison, the UK Parliament has historically played a limited and often indirect role in authorizing the use of force. From the vote on the Iraq War in 2003 onwards, however, the UK Parliament's role has increased to the point where, in August 2013, the defeat of a Government motion seeking approval for the use of force undermined efforts to build an international coalition to intervene in the ongoing Syrian conflict. Whilst debate regarding this shift has hitherto concentrated on the degree to which parliamentary oversight of the war prerogative is desirable, in this article we consider what Parliament's evolving role heralds for the general relationship between domestic and UN mechanisms. We challenge the underlying assumption that Parliament's interventions mark an indisputably positive development in constraining the use of force. When coupled with the focus upon the doctrine of humanitarian intervention which has accompanied many controversial exercises of UK military force since the end of the Cold War, the involvement of Parliament in the decision-making process risks hollowing out UN Charter safeguards. Successive UK Governments have acquiesced to the extension of Parliament's role, with the effect of shifting the locus for legitimating uses of force away from UN institutions, where the UK cannot control the actions of other States, and into a domestic sphere which is susceptible to executive influence.