A P.O.U.M. recruiting poster. Although not definining itself as Trotskyist, the P.O.U.M. did have foreign Trotskyists, including Harry David Milton, fighting in the ranks of its militia units. The P.O.U.M. leader, Andreu Nin, was killed by Stalinists in the wake of the May events in Barcelona. His exact fate remained unknown until much later, and the slogan "Where is Nin?" appeared on walls throughout Republican Spain.