I presume your question is in reference to The Great War, which we now call World War I.
The days leading up to the Great War (World War I) were full of military buildup, military planning, and countries lining up with one another in alliance systems. The Triple Entente had Britain, France and Russia as allies. In between those countries were the members of the Triple Alliance: Germany, Austria-Hungary, and Italy, with Germany as the leading power in that group.
Because of the alliance system, Germany assumed that if a war began, it would need to fight on two fronts -- west and east. Germany assumed it would be fighting against both France (on its western border) and Russia (on its eastern border) if war broke out.
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.
