World War I lasted from July 28, 1914, to November 11, 1918. This means the war lasted for about 4 years and 3.5 months. The war began when Austria- Hungary declared war on Serbia on July 28, 1914, and ended when Germany signed the armistice on November 11, 1918, which caused the fighting to stop at 11 a.m. that day, marking Armistice Day.