The great flowering of American modern art since 1945 has ori- gins that go back much earlier-to the years after World War I, when writers, critics, and artists argued over the cultural health of the nation. Europe beckoned, and many, including Ernest Hemingway, fled to Paris. But others remained in Manhattan to do battle in little magazines such as Soil and Broom. Was Amer- ica in the 1920s a sinkhole of crass "commercialism," or were the new industrial machines, consumer gadgets, and...