In The Battle of the Marne, the distinguished WWI scholar Holger Herwig argues that the opening battle of the war was perhaps the most significant land battle of the 20th century. At the very least, the Marne was the most decisive land battle since Waterloo (1815).
First, the scale of the struggle was unheard of before 1914: France and Germany mobilized roughly 2 million men each, Britain some 130,000. During the momentous days between 5 and 11 September 1914, the two sides committed nearly 2 million men with 6,000 guns to a desperate campaign along the Marne River on a front of just 200 kilometers between the "horns of Verdun and Paris."
Second, the technology of killing was unprecedented. Rapid small-arms fire, machine guns, hand grenades, 75mm and 77mm flat-trajectory guns, 150mm and 60-pounder heavy artillery, mammoth 305mm and 420mm howitzers, and even aircraft made the killing ground lethal.
Third, the casualties ("wastage") suffered by both sides were unimaginable to prewar planners and civilian leaders alike: 200,000 men per side in the Battle of the Frontiers around the hills of Alsace-Lorraine and the Ardennes in August, followed by 300,000 along the chalky banks of the Marne in early September. No other year of the war compared to its first five months in terms of death.
Fourth, the immediate impact of the draw on the Marne was spectacular: the great assault on Paris had been halted and the enemy driven behind the Aisne River. France was spared defeat and occupation. Germany was denied victory and hegemony over the Continent. Britain maintained its foothold on the Continent.