This article looks at the effect of public support on the counter terrorism initiatives of three different “Western democracies” – the British Government response to the Provisional Irish Republican Army, the Italian Government’s response to the Red Brigades, and the Canadian Quebecois Government’s response to Quebec Liberation Front. These examples were chosen as they are three examples of Western democracies dealing with nationalist/left-wing violence at around the same time. This article argues that public support can provide a mandate for action but that it can also set boundaries for the measures governments and their non-state adversaries can legitimately use. As the Italian and Canadian examples illustrated, when terrorist organizations escalate to a quantity or quality of violence that their supporters deem excessive in relation to the issues at stake, a rapid collapse of legitimacy may follow. Such “critical junctures” create opportunities for government forces to gain the upper hand as terrorist organizations that become socially isolated can be pursued harshly at considerably lower risk of sparking claims of government overreaction or repression. In the British example, the Government pursued counterterrorism campaigns while not enough attention to the way in which its policies could affect their opponent’s popular backing, which had a counterproductive results. Fluctuations in the quantity and quality of public support for either side can expand or contract these boundaries, potentially instigating marked changes in conflict-related violence levels and affecting the efficacy of government countermeasures.