The Battle of the Frontiers was a series of running engagements between the combatants on the western front at the beginning of World War I. The French, led by Marshal Joffre, were attempting to realize their preconceived offensive strategy, Plan XVII, by invading Alsace Lorraine. However, the Germans gained the upper hand and drove rapidly into northern France within 20 miles (32 km) of Paris, before the Allies regrouped south of the River Marne. While the Germans held the advantage, they failed to make a decisive breakthrough; this has been attributed to several factors. Firstly, the speed of the German advance outran their supply lines. Secondly, the German Chief of Staff, Field Marshal von Moltke, diverted a significant portion of his armed forces to the eastern front to counter the Russian invasion of East Prussia. Finally, the Great Retreat never became a rout: Allied spirit remained unbroken as the subsequent first Battle of the Marne demonstrated.