June 28, 1914: Archduke Franz Ferdinand, heir to the Austrian throne, and his wife, visit Sarajevo in Bosnia. A lone assassin shoots and kills them both. Austria believes the killer is linked to the Serbian nationalist movement.
July 23: Austria-Hungary, with the backing of Germany, delivers an ultimatum to Serbia. The Serbs offer to submit to arbitration, but also begin to mobilize their army.
July 25: Austria-Hungary cuts diplomatic ties with Serbia and begins to mobilize.
July 26: Britain tries to convene a conference of the major European powers to resolve the situation. France, Italy and Russia agree to take part. Germany says no.
July 28: Austria-Hungary declares war on Serbia.
July 29: Britain calls for international mediation. Russia urges German restraint, but then begins partial troop mobilization as a precaution. The Germans warn Russia and begin to mobilize themselves.
July 30: Austria shells bombard Belgrade, the Serbian capital.
July 31: Russia begins full mobilization. Germany demands that it stop.
Aug. 1: Germany declares war on Russia. France and Belgium begin full mobilization.
Aug. 3: Germany declares war on France and invades neutral Belgium. Britain delivers an ultimatum to Berlin demanding withdrawal from Belgium. Germany ignores it.
Aug. 4: Britain declares war on Germany. The declaration is binding on the British Empire, including Canada, Australia, New Zealand, India and South Africa.
Aug. 6: Austria-Hungary declares war on Russia.
Aug. 7: First British troops land in France. About 120,000 men of the regular British Army form the British Expeditionary Force under Field Marshal John French.
Aug. 7-24: Seeking a quick victory, the French invade Alsace and Lorraine, to be met by German counterattacks. The French suffer huge losses, including 27,000 men killed in a single day, the worst one-day toll in the history of the French army. Meanwhile, the German right wing is beginning to move across Belgium towards northeastern France. The plan calls for that wing to sweep around Paris like a huge swinging door to catch the eastward-facing French armies from behind and pin them against the frontier.
Aug. 12: Britain and France declare war on Austria-Hungary, which invades Serbia.
Aug. 17: Russia invades Germany.
Aug. 23: The BEF encounters the invading Germans at the Belgian city of Mons. After inflicting heavy casualties, the vastly outnumbered British begin a fighting retreat.
Sept. 5-12: The weary Germans change their plan and turn south before passing Paris. In doing so, they expose their flank. In the Battle of the Marne the French and British halt the great German sweep across northern France, squelching their hope of a lightning victory.
Sept 17-Oct. 19: Both sides try to outflank each other, side-stepping north in what was called "The Race to the Sea." When they finally reach the North Sea, Europe is essentially bisected by a front that stretches to the Swiss border. Both sides begin to dig in and the infamous Western Front of trench lines is born.