Just a note to recommend Holger Herwig’s newish book about the first month of the War. It covers more than just the September Battle of the Marne, but gives a clear account of the War’s beginnings, and of the savage actions and bloody confusions during the Battle of the Frontiers in August 1914.
Drawing on French and German archives (including some only available after the reunification of Germany), Herwig shows the two sides confusedly trying to make sense of one another, as plans founder against counter-plans in the chaos of war. He stresses the part played by chance, miscommunication and personalities as soldiers of both sides made fundamental errors in understanding what the other side was doing.
The most grievous errors were German. The Kaiser and Moltke implemented Schlieffen’s plan, but then diverged from it. Chances were missed and lack of communication between the various armies, and between the armies and headquarters (in Luxemburg, a long, long way from the battlefield) meant that forces were not coordinated when victory was a definite possiblity. It was this lack of communication that made the pessimism of Hentsch, acting as Molke’s envoy, so persuasive and infectious. Only he seemed to have an overview of the situation, and when he advocated withdrawal from the Marne, the whole impressive German army withdrew, to what would become the static trench lines of the next four years.
Joffre, in contrast, kept his troops together, and radiated aggressive confidence, even when things were at their worst. His victory on the Marne was less a tactical one (casualties, on a horrendous scale, seem to have been similar for both sides) but one of strategy. In the face of French attacks, the Germans lost their nerve.
It has been suggested (and I think Herwig agrees with this) that by losing the Battle of the Marne the Kaiser lost the War. His whole strategy had been based on an early end in the West (the Schlieffen Plan was supposed to have been completed in 39 days) and once he got bogged down into trench stalemate he had missed the chance of a decisive victory.
This book tells the story well (though I could have done with more maps).