When Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian empire, and his wife are killed by Bosnian Serb nationalist Gavrilo Princip, it ignited a series of political, diplomatic and military events that would turn what started as a local conflict in south-east Europe into a world war. As tensions mounted in the region, countries formed alliances to protect themselves from potential invasion by enemies on their borders. Austria-Hungary and Germany signed an agreement to support each other in a European war, while France and Russia reached a Triple Entente with Britain.

Taking advantage of the fear of potential American entry into the war, the Germans send the coded Zimmermann Telegram to Mexico offering Texas, New Mexico and Arizona back in exchange for backing Germany in the war. When the telegram is intercepted and decoded, Woodrow Wilson calls for an end to American neutrality and announces the US will enter the war on the side of the Allies.

The first battles are bloody as entrenched armies take up positions along the front lines in trench warfare. At the Battle of Ypres, British forces shatter the German lines of defence. This victory helps to stop the planned German invasion of France and sets the stage for the Battle of the Marne, which proves to be a turning point in the war.

Despite being outnumbered, the Allies eventually come into their own as they deploy better training and doctrine. They are aided by technological innovations including tanks and airplanes as well as lethal new weapons like poison gas. By the time the war ends in 1918, more than 16 million military personnel and civilians will have died.