Labor activists have called for greater international coordination among trade unions in response to the assault on organized labor by global capital, but such coordination faces many hurdles. Under what conditions can unions overcome those barriers and coordinate effectively to achieve campaign goals? I examine this question through a comparison of European-level international solidarity with Portuguese, Greek and English affiliates of the International Dockworkers Council involved in labor disputes. The divergent outcomes of otherwise similar cases reveal the critical role of politics and strategy at different scales and sites of union organization in determining the successful exercise of labor internationalism.