Hundred Days, the period between March 20, 1815, and June 28, 1815. On March 20 Napoleon I returned to Paris from exile at Elba. The French army deserted Louis XVIII and flocked to the former emperor. Alarmed, the other European nations formed a coalition and assembled troops along the borders of France. Napoleon was defeated by the coalition forces in the Battle of Waterloo on June 18 and abdicated June 22. On June 28 Louis XVIII returned to France to assume the throne.