The ‘Arab Spring’ started as a protest in a small town in Tunisia. Protest then spread from village to village and town to town before emerging as a country-wide movement. How does protest emerge and what enables it to spread? This paper examines how the social structure (the network) of a population affects the ability of collective action to spread. We propose a model of the spread of protest. We consider how adding or removing social connections within a network can help or hinder the spread of protest. We find that for any network, any potential social tie can be categorized into one of exactly two types: either a ‘strong tie’ or a ‘weak tie’. We find that adding a ‘weak tie’ improves the spread of protest when protest conditions are ‘mild’ but hinders protest when conditions are ‘severe’. In contrast, adding a ‘strong tie’ hinders protest when conditions are ‘mild’ but helps protest to emerge and spread when conditions are ‘severe’.