Laura Cumming explores one of the most exciting periods of art history as the exhibition 1900: Art at the Crossroads opens at the Royal Academy. One hundred years later, the work of many now less well known artists is being reassessed alongside established masters. What can a cross-section of the art of a single year reveal about the directions art was to take in the century following? Plus a review of Three Seasons, the first American film to be shot in Vietnam since the war, which explores a country in transition between traditional ways of life and the modern values of westernisation and progress.