The Battle of the Frontiers was a handful of battles fought between Germany and the Allied force of France and the United Kingdom in the early days of World War I.

The German high command had deployed a modified version of the Schlieffen Plan, which called for a German invasion of neutral Belgium in order to achieve an envelopment of French troops massed at the France-Germany border. The strong belief was that even though France and Russia had a mutual defense pact as part of the Triple Entente (the third member of which was the United Kingdom), Russia would not be able to mobilize its forces in time to distract the German invasion of France, which Alfred von Schlieffen had estimated would take only six weeks to achieve.

Russian forces deployed much more quickly than anyone else thought, invading East Prussia on Aug. 7, 1914, just a few days after Germany had invaded Belgium. The German response was immediate, shunting troops to the east, including taking them from the force that had advanced into Belgium.

Battle of Mulhouse

On the same day that Russian troops invaded from the east, French troops invaded Alsace, which France had ceded to Prussia after losing the Franco-Prussian War 40 years earlier. The Battle of Mulhouse lasted for several days, during which French troops succeeded in taking the town, after the fighting descended into house-to-house fighting. The German retreat left behind 3,000 prisoners and two dozen big guns.

Battle of Haelen
During this same time period, German forces fought a cavalry battle against Belgian forces near the small town of Haelen. After a brief back-and-forth, Belgian defenders carried the day, by dismounting and turning rifle fire onto the advancing German cavalry. It was one of few Belgian victories during the war.

Battle of Lorraine
While French and German troops were still consisting the area around Mulhouse, a French offensive in the south moved in the direction of Sarrebourg. After a series of pitched battles in nearby Cirey and Donon, French troops captured the city of Sarrebourg. The end was basically beginning, as the armies returned mostly to their starting positions.

Battle of the Ardennes
Germany launched a counteroffensive against the French advance into Lorraine; at the same time, French troops massed into the Ardennes Forest, crossing into Belgium, which Germany had already mainly occupied. German troops had the upper hand at the end of this three-day battle, forcing a French retreat.

Battle of Charleroi
French forces and members of the British Expeditionary Force teamed up against Germany in this three-day battle. The result was a German victory, forcing the evacuation of Namur and a retreat of French forces from the Ardennes.

Battle of Mons
On the same day that the Battle of Charleroi ended, U.K. troops and German troops clashed at Mons. It was the first major military action of the British Expeditionary Force, and it ended with the numerically superior German Army forcing U.K. troops to retreat.

The main result of all of these battles was an overall French retreat and an overall German advance. German troops continued to press their advantage into France, targeting the capital, Paris. The next major clash was the First Battle of the Marne.