World War I was fought from July 28, 1914 to Nov. 11, 1918. The war ended with nearly 10 million dead and 20 million wounded. The German, Austro-Hungarian and Ottoman empires collapsed, leaving geopolitical divisions that remain as of 2014.
The spark that ignited WWI was the assassination of Austrian Archduke Franz Ferdinand in June 1914 by Serbian nationalists. Soldiers on both sides dug an extensive network of trenches throughout France to protect themselves from machine-gun fire and artillery. The tank was invented during WWI as a mobile gun platform to break through enemy lines. WWI was one of the world's most destructive conflicts, with over 37 million casualties.