The early weeks of World War I were just the opening act in a conflict that would continue for more than four years. The countries that declared war in the summer of 1914 could not know that the fighting would not end until November 1918. The first weeks of the war saw the combatant nations mobilizing their forces and opposing armies clash for the first time.

Declarations of War

The war spread across Europe during the summer of 1914 as the combatant countries declared war on each other over a period of almost a month. A complex network of political treaties drew countries into conflict with each other. What began as a localized war between Austria-Hungary and its neighbor Serbia, declared on July 28, had grown into a much larger conflict involving Germany, Russia, France, Japan and the British Empire by the end of August.

Mobilization

The combatant nations quickly began to mobilize their troops. Some countries, such as Germany and France, maintained large pre-war armies -- Germany’s, for example, stood at 1.9 million troops in 1914. As a result, these countries could call on large numbers of trained men quickly. However, other countries, including the United Kingdom with its 450,000-strong army, had to call for volunteers to join the military. A recruitment campaign encouraged an estimated 500,000 men to join the British army by mid-September 1914. At the peak enlistment period in late August 1914, 30,000 British men joined every day.

Troop Movements

The first weeks of the war witnessed large-scale troop movements across western Europe. German soldiers advanced quickly through Belgium and northern France, reaching a line 30 miles from Paris by early September. Meanwhile, on the Eastern Front, German troops defeated their Russian counterparts at the Battles of Tannenberg and the Masurian Lakes in late August, while in the month’s last week the Austrian army was heavily defeated by the Russians in eight days of fighting at the Battle of Lemerg in present-day Poland.

Fighting

Fighting was fierce and losses heavy in some of these early battles. The French army lost heavily in defending Paris, sustaining 250,000 casualties in the Battle of the Marne in early September. Precise figures do not exist, but it’s estimated that the German army lost a similar number of men in the battle. The Eastern Front was no less bloody: at the Battle of Tannenberg in September 1914, the Russian army lost 250,000 men, with a further 92,000 taken prisoner.