Archduke Franz Ferdinand heir to the throne of the Austro-Hungarian Empire, and his wife are assassinated in Sarajevo.

Emperor Franz Joseph of Austria-Hungary declares war on Serbia.

As an ally of Serbia, Russia announces full mobilization of her armed forces.

Germany mobilizes her armed forces and declares war on Russia.

Photo: German soldiers celebrating start of war

Germany declares war on France.

Germany declares war on neutral Belgium and invades in a right flanking move designed to defeat France quickly. As a result of this invasion, Britain declares war on Germany.

Photo: German troops crossing into France from Belgium

Austria-Hungary declares war on Russia.

"The Battle of the Frontiers" -- 27,000 French soldiers die on this single day in an offensive thrust to the east of Paris, towards the German borders.

German army, led by Erich Ludendorff and Paul von Hindenburg achieves its greatest victory of the war on the Eastern front against Russia at the Battle of Tennenberg.

Photo: Generals Ludendorff and von Hindenburg with Kaiser Wilhelm II

First Battle of the Marne halts German invasion in France.

First trenches of the Western front are dug.

Photo: French soldiers building a trench

Unofficial Christmas Truce declared by soldiers in the trenches along the Western Front.

Photo: Western Front trench covered with snow, modern footage