ABSTRACTThis study identifies three types of workplace union strategy in the development of cross‐border relations within North American and European multinational companies: defensive isolation, risk reduction and proactive solidarity. Qualitative case studies of MNCs with operations in Canada and Mexico indicate that the nature and intensity of participation in cross‐border trade union alliances are shaped by the union dynamic at the local, national and international levels. A combination of greater workplace union power resources, notably discursive capacity, and of strong supportive approach of the national union, notably dedicated resources and space for bottom‐up initiatives, contributes to proactive solidarity strategies towards international union networks. The absence of these factors is associated with risk reduction and defensive isolation strategies.