The morning of June 28, 1914 dawned bright for most Europeans. By sunset a geopolitical cataclysm loomed. World War I demonstrated the importance of saying no. Any of the great powers could have stopped the march toward war. America could have refused to join the parade after it started. The world would have been a better place had one or all done so. Today, Washington is filled with routine proposals for new interventions: bombing campaigns, foreign invasions, and military occupations. Most seem unlikely to trigger a new world war. But a century ago no one expected an assassination in a distant Balkan province to do so either. That is reason enough for Americans to make war truly a last resort.