In 1966, Ralph Schoenman persuaded Bertrand Russell to convene his war crimes tribunal to judge United States involvement in the Vietnam War. Horowitz would write three decades later that he had political reservations about the tribunal and did not take part. He described the tribunal's judges as formidable, world-famous and radical. They included Isaac Deutscher, Jean-Paul Sartre, Stokely Carmichael, Simone de Beauvoir, Vladimir Dedijer and James Baldwin. In January 1966, Horowitz, along with members of the Trotskyist International Marxist Group, formed the Vietnam Solidarity Campaign. The Vietnam Solidarity Campaign organized a series of protests in London against British support for the Vietnam War.