Germany's plan

Germany's plan (named the Schlieffen plan) to deal with the Franco-Russian alliance involved delivering a knock-out blow to the French and then turning to deal with the more slowly mobilized Russian army. Rather than attack France directly, it was deemed prudent to attack France from the north. To do so, the German army had to march through Belgium. Germany demanded this free passage from the Belgian government, promising that Belgium would be Germany's firm ally if this was agreed to. When Belgium refused, Germany invaded and began marching through Belgium anyway, after first invading and securing tiny Luxembourg. It soon encountered resistance before the forts of the Belgian city of Liège. Britain sent an army to France, which advanced into Belgium.
The delays brought about by the resistance of the Belgian, French and British forces and the unexpectedly rapid mobilization of the Russians upset the German plans. Russia attacked in East Prussia, diverting German forces intended for the Western Front. Germany defeated Russia in a series of battles collectively known as the (second) Battle of Tannenberg, but this diversion allowed French and British forces to finally halt the German advance on Paris at the First Battle of the Marne (September 1914) as the Central Powers were forced into fighting a war on two fronts.