Abstract In 1967, amid the U.S. war in Vietnam, distinguished leftist intellectuals and activists gathered in Sweden and Denmark to establish the first citizens’ tribunal. Initiated by the philosopher Bertrand Russell, the purpose of the International War Crimes Tribunal (iwct) was to put the U.S. government on trial for its military actions in Southeast Asia. This article has two objectives. First, it argues that this activist endeavour was embedded in a larger internationalist movement against Western imperialism, namely Tricontinentalism, of which the war in Vietnam was a connecting factor. Second, the article investigates the tribunal as a manifestation of the solidarity that the New Left in the West maintained with revolutionary movements in the Third World. It aims to show the utility of the theoretical concept of political solidarity, most thoroughly elaborated by Sally J. Scholz, for the global history analysis of social justice movements. At the same time, it contends the necessity of additional parameters when studying manifestations of political solidarity in a post- and settler-colonial context, drawing on new scholarship on colonial-sensitive solidarity. By carving out the specificities of this case of activism on the theoretical ground, this investigation also highlights the importance, advantages, and pitfalls of political anti-imperialist solidarity.