How do political protests transform into a social movement? Analysing Czech anti-war protest events between 2002 and 2009, this study aims at identifying particular mechanisms through which the social-movement mode of coordination was established out of a fragmented field of activism through consecutive protest campaigns. The study shows how broadening collective identity and creating instrumental relationships in times with the opening of political opportunities is followed by the institutionalisation and further expansion of protest cooperation during a period of heightened political threats, which establishes a new mode of protest coordination.