In 1914, the assassination of an Austrian archduke set off a disastrous four-year-long conflict involving dozens of countries with battles taking place in all parts of the world. World War I was the first to use planes and tanks as well as deadly gases that left soldiers blinded or "shell shocked" (a condition now called Post Traumatic Stress Syndrome). There were battles that lasted for months with opposing troops fighting from rat-infested trenches, battles that often ended in a hollow victory with only a small area of land retaken.

What Was World War I? -- Rival Empires -- One Wrong Turn in Sarajevo -- A Different Kind of War -- Trench Warfare -- War in the Skies -- War at Sea -- The United States Enters the War -- Women in the War -- Here Come the Doughboys! -- The Treaty of Versailles -- Legacy -- Timelines.