Legislatures in separation of powers systems like the US are often portrayed as having far greater capabilities and willingness to change defence policy than are parliaments in Westminster systems. This paper uses principal–agent models and hypotheses on legislative will to review the role of defence committees in the US Congress and Britain's parliament during each country's most recent, significant change in civil–military relations. Congressional committees drafted the 1986 Goldwater–Nichols Act over the objections of the president, fundamentally changing US civil–military relations. We would expect the British House of Commons to be at the opposite end of the spectrum, unable and unwilling to act without the prime minister's blessing. At first glance, this is indeed what happened during Britain's 2011 Defence Reform effort. Parliament took no concrete, independent action. A closer examination, however, suggests that parliamentary committees helped set the agenda for the 2011 reforms. These results point to the need to carefully assess both legislative capabilities and will when examining the role of legislatures in foreign policy, as well as the indirect means by which parliaments affect security policy.