Another domino on the way to war fell on August 3rd when Germany declared war on France. France was allied with Russia, who Germany had declared war on two days earlier, and it was generally understood that France would soon declare war on Germany. Thus, Germany planned to preempt a French attack by invading Northern France through Belgium. To this end, the German Army mobilized against France and the two nations entered a state of war.
The initial German invasion of France went very well. Although the Belgian fortifications held out longer than the German planners expected, the offensive was able to occupy the whole of Belgium and sweep into France. Germany occupied the most industrialized region of France within months of the outbreak of the war, and forced the French Army back to the Marne. At the Battle of the Marne, however, the overstretched German forces were stopped and forced to give up some of the ground they gained. The Western front then settled into a stalemate characterized by costly Allied offensives to retake French and Belgian territory, generally to no avail. The invention of chemical weapons, tanks, and introduction of planes all failed to break the deadlock. It would take until 1918 and the German Spring Offensive to restore mobility to the front. Using infiltration tactics, prototype sub machine guns, and concentrated attacks, German storm troopers broke Allied trenches and pushed their forces back. The Germans advanced across all of the territory the Allies had recaptured over the past four years, and again reached the Marne. The Second Battle of the Marne would see the Germans stopped yet again. This time, however, there would be no drawn-out stalemate as German food shortages and the influx of American troops led to a collapse of the German Army and the armistice on November 11th.
That a war in the Balkans led to a German declaration of war on France shows the perils of unconditional alliances. The French alliance with Russia drew France into the Russian war with Austria, which it itself had only entered because of its commitment to Serbia. While international alliances can certainly prevent war when nations are reasonable, in an era of nationalism and militarism, they only served to expand its scope.