It is axiomatic that high-risk activism requires solidarity if social movements are to have success in struggles against powerful adversaries. However, there is little research that attempts to gauge the impact of various types, limits, or breakdown of solidarity directly and systematically. Drawing from historical political economy, cultures of class formation, and social movement outcome literatures, we address the question of solidarity’s impact across dimensions and at various levels of scale (i.e., at the point of production or firm level, local community, and wider society) by analyzing the outcomes of more than 4,500 strikes during the late-nineteenth-century rise of US industrial capitalism. We find that while strike solidarity at the point of production is necessary, it is not sufficient for success. Disruption costs that strikers seek to impose to gain leverage can be significantly reduced by the countertactic of hiring strikebreaking replacement workers recruited from the local community or imported from beyond. We also find that the urban regime of strike policing matters by moderating the impact of strikebreakers. The most powerful predictor of strike outcomes is employer use of replacement workers, signaling the key to undermining working-class strike solidarity directly pits the working class against itself. Intraclass solidarity is necessary for the success in interclass struggle but needs to extend beyond the struck firm implicating the importance of solidarity of the surrounding community and wider social factory. We discuss the implications of these findings for understanding the historic formation of the US labor movement and its present predicament.