Robertson rebelled against his party's Government on three occasions, most notably on a rebel Amendment vote (Division No. 117) prior to the main Declaration of War – Iraq vote (Division No.118). This Amendment said: This House "believes that the case for war against Iraq has not yet been established, especially given the absence of specific United Nations authorisation; but, in the event that hostilities do commence, pledges its total support for the British forces". However, just 45 minutes later he backed the Government on the main vote authorising "all means necessary to ensure the disarmament of Iraq's weapons of mass destruction", in effect supporting the 2003 Iraq war. He later referred to these events in his September 2013 newsletter published online, in which claimed that he was "one of the 'rebels' who voted against the Iraq War in 2003." During the campaign for the 2015 general election, he participated in a hustings meeting in the Drumchapel district of his Glasgow North West constituency. In response to a question from a member of the public, he repeatedly denied that he had voted for the Iraq War. The SNP candidate, Carol Monaghan, again asserted that Robertson had voted for the Iraq War, to which he responded "not true". These denials led to heated arguments with members of the public who accused him of being a liar. Robertson was defeated by Monaghan at the subsequent election.