After the Allied victory of the Marne, their pursuit was slow, due to exhaustion, giving the Germans plenty of time to build defences along the heights overlooking the Aisne River. Once they arrived on the 13th, the Allies launched frontal attacks across the river which were initially successful, but counter-attacks beat them back almost to the start line. Fierce fighting then ensued along the line eventually descending into artillery duals, and convincing both sides of the futility of frontal assaults against prepared positions and modern weapons. The Allies thus decided to try a set of flanking movements in the direction of the coast which became known as the 'Race to the Sea'.