Germany Declares War on France
On the afternoon of this day in 1914, two days after declaring war on Russia, Germany declares war on France, moving ahead with a long-held strategy, conceived by the former chief of staff of the German army, Alfred von Schlieffen, for a two-front war against France and Russia. Hours later, France makes its own declaration of war against Germany, readying its troops to move into the provinces of Alsace and Lorraine, which it had forfeited to Germany in the settlement that ended the Franco-Prussian War in 1871.
After 1870 European conflict was averted largely due to a carefully planned network of treaties between the German Empire and the remainder of Europe—orchestrated by Chancellor Otto von Bismarck. He especially worked to hold Russia at Germany's side to avoid a two-front war with France and Russia. With the ascension of Kaiser Wilhelm II as emperor, Bismarck's system of alliances was gradually de-emphasized. For example, the Kaiser refused to renew the Reinsurance Treaty with Russia in 1890. Two years later the Franco-Russian Alliance was signed to counteract the force of the Triple Alliance. In 1907, the British Empire joined France and Russia, signaling the beginning of the Triple Entente.
Wikipedia: World War I