In May 2014, Stewart was elected by MPs from all parties as Chairman of the Defence Select Committee. He was the youngest Chair of a select committee in parliamentary history, as well as the first MP of the 2010 intake to be elected to chair a committee. In this capacity, Stewart argued strongly for a more vigorous response to Russian aggression in Ukraine. The committee also argued that Britain's commitments to Iraq and Syria were "strikingly modest" and that more should be done. Under Stewart's chairmanship, the committee produced a report in favour of the proposals for a Service Complaints Ombudsman and also secured an amendment extending the powers of the Ombudsman.