The Schlieffen Plan proposed that in the event of war, Germany would quickly attack and control the western front against France, after which most German troops would be deployed to the eastern front against Russia.
Context/explanation:
The military plans laid before World War I presupposed a major war between the countries which were tied together with alliances. Because the Triple Entente had Britain, France and Russia as allies, Germany thought if a war began it would need to fight on two fronts -- west and east. So German Field Marshall Alfred von Schlieffen drew up war plans that said attack France first, quickly, and then hold that territory while deploying forces to contend with Russia in the east. In 1914, when Russia mobilized troops to come to the aid of Serbia against Austria-Hungary in 1914 (after the assassination of Austria's archduke by Serbian radicals), Germany declared war on Russia. And when Germany went to war, the first thing it did was to march through Belgium to go and attack France. Up to that point France had not had involvement in the conflicts that had begun in the Balkans. Thus the war spread and quickly became a more global conflict.