In this paper, I suggest a novel mechanism through which issue linkage in international relations can arise, termed "defensive issue linkage": As the scope of one issue area expands, it begins to touch upon an increasing number of previously unrelated issues, both domestic and international. Actors operating in these previously unrelated issue areas become worried about potential interference and begin mobilizing. Thus mobilized, these interest groups seek carve-outs to preserve their traditional policy space, and attempt to exert more direct influence on the policy channels of the expanding issue area. To demonstrate this mechanism, I focus on the example of trade and environmental policy, where political linkages have rapidly increased since the early 1990s. Drawing on historical evidence from the U.S. and interviews conducted in Europe, I show that environmental civil society groups have been important drivers of the emerging political linkage between the environment and trade. Further, environmental groups have tended to mobilize when trade policy expanded in scope, often citing threats these new trade rules pose to environmental policy as their primary concern. In addition to this qualitative evidence, a quantitative analysis of all preferential trade agreements signed between 1989 and 2016 finds that the presence of environmental civil society groups is positively associated with the prominence of environmental content in trade agreements, and that this relationship is moderated by trade agreements' economic scope.