Bangladesh, Oct. 19 -- The growing convergence between the radical left and Islamist political movements is becoming a contentious issue in Western democracies, raising questions about ideological alliances, political polarization and the future of liberal institutions. As France confronts another wave of violent unrest, a Washington-based political watchdog is warning that similar ideological coalitions could influence the United States ahead of the November midterm elections. Supporters of this warning describe the phenomenon as the “Red-Green Alliance”-a controversial term for cooperation between Marxist-inspired political movements and Islamist organizations.

The central question is not whether every left-wing activist or...