In the 1960s Weiss became increasingly politically radical, taking stands for revolutionary Cuba and against US intervention in Vietnam and visiting both countries. In 1966 he visited the United States together with the West German writers group "Gruppe 47". During a conference at Princeton University he denounced the US war against North Vietnam which seems to have scandalized his German colleagues more than his US hosts. In 1967 he participated in the anti-war Russell Tribunal in Stockholm and in 1968 he joined the eurocommunist Swedish Left Party (VPK). During the same year he also visited North Vietnam and published a book about his trip.