At least textbooks used to say so. And then American art got into trouble. The textbook history has changed, from Irving Sandler's The Triumph of American Painting to Robert Hughes's nostalgia. The Whitney looks provincial again, especially in a Biennial without artists from abroad. When it proclaims these years an American century , watch out.
Postmodernism sometimes makes it public enemy number one.
That can mean forgetting how many fits and starts art has had along the way. It can mean assimilating pretty much everything to postwar American painting. One knows art is in trouble when it claims to speak for everyone. Think of Disney taking over the Broadway theater. And Modernism, including Abstract Expressionism, is up against it these days.