In Greenberg's view, after World War II the United States had become the guardian of 'advanced art'. He praised similar movements abroad and, after the success of the Painters Eleven exhibition in 1956 with the American Abstract Artists at New York's Riverside Gallery, he travelled to Toronto to see the group's work in 1957. He was particularly impressed by the potential of painters William Ronald and Jack Bush, and later developed a close friendship with Bush. Greenberg saw Bush's post-Painters Eleven work as a clear manifestation of the shift from abstract expressionism to Color Field painting and Lyrical Abstraction, a shift he had called for in most of his critical writings of the period.