What factors account for labor strategies in global industries? While some scholars point to economic factors and others look to political opportunity structures, an examination of union actions in the Central American apparel export industry over a 14-year period suggests that activists’ historical experiences and ideological orientations also strongly influence union dynamics. Left-oriented unions tend to form unions through transnational activism whereas conservative unions most often turn to plant-level cross-class collaboration. Moreover, these two union strategies are interconnected. Successful transnational activism facilitates conservative union formation through a “radical flank” mechanism; the threat of left-union organizing motivates employers to accept unionization by conservative unions to block left unions from gaining influence in the plant. To examine these arguments, this article employs pooled time-series statistical analysis, structured interviews with labor organizers, and process tracing that draws on nine months of field research in Honduras and El Salvador.