Since the disappearance of the linear devices of the Alliance's niches, a notion has been stirring up a lot of minds in terms of tactical thinking, that of the so-called gap spaces. The example of the Battle of the Marne is edifying in this respect.

During the Battle of the Marne, it was an interval of 120 km deep by 60 km wide, which occurred in the German scheme between the two armies of the right wing that caused the decision to retreat.

How was this "gaping space" created and how did the German command manage to solve the problem it was facing?

The creation of the gap between the 1st and ^{2nd German} armies

On the morning of 6 September 1914, the first echelon corps of the German right wing, in pursuit of the French and British armies and composed of the 1st (von Klück) and 2nd (^{von Bülow)} armies^, crossed the Marne and ^its southern ^tributaries, the Grand and Petit Morin. Paris is masked and von Moltke thinks to give the stunt to a French army defeated at the borders. He had previously entrusted the coordination of the armies of the right wing to Bülow, the oldest of the army commanders, who, as a result, was truly exercising the responsibilities of army group commander. It was within this framework that, since the French offensive return of the 5th ^{Army to Guise} at the end of August, Bülow had ordered Klück to advance in the refused echelon in relation to him, so as to cover him, if necessary. But, relying on vague directives from the O.H.L. (the German High Command) which let him off the hook, the commander of the 1st Army ^took no notice and continued his own progression, without taking into account Bülow's request for cover. His heads were even ahead of those of the 2nd Army at the ^{beginning of} September.

It was in this situation that Joffre launched a counter-attack by ^Maunoury's 6th ^army on Klück^'s exposed flank from the Ourcq valley, while dealing a huge blow ofto stop the German advance along the whole front from the south of the Marne to Verdun, while ensuring the "locking" of the Grand Couronné and the trouée de Charmes.

Surprised by this action, which he had never expected and noting that it could constitute a serious threat, Klück, in the middle of the battle, broke two army corps from the south to the north-west of his position. But in doing so, if he kept to the north, he lost contact with the 2nd ^army to his east. Deprived for the first time in the campaign of his freedom of action, his right wing blocked in its progression, the German command was unable to manoeuvre its marching wing. Worse, because of Klück's indiscipline, the two right-wing armies progressed in a divergent manner, so that instead of supporting each other, they found themselves disjointed and, when Klück was killed, the two armies were unable to move forward.When Klück was forced to take forces from his leading echelon to counter Maunoury's attack, he irreparably created a gap between himself and Bülow. The breach thus created is 120 kilometres deep and about 60 kilometres wide.

The German command's consideration of the problem

Confronted with this mortal danger, the German Chief of Staff, von Moltke sent his head of the Intelligence Bureau, Saxon Lieutenant Colonel Hentsch, to the army commanders with full delegation of authority. It was he who decided the withdrawal of the German right wing, which was to lead to the disappearance of this breach in the short term.

But beforehand, Moltke had taken precautionary measures; in particular, since it was impossible for him in the immediate future to absorb this breach, he had decided to mask it with a cavalry curtain, a mission entrusted to the cavalry corps commanded by von der Marwittz. This was not a common use of large cavalry units, but faced with an unprecedented problem, Moltke was able to find a palliative solution. During the retreat to the Aisne, it was this cavalry curtain that masked this real "gaping space" between the two German armies of the right wing. But even so: Joffre had managed to stop the German advance, then to make his opponent retreat, but after a month of exhausting campaign during which the losses were heavy, he no longer had the slightest tool for exploitation; the French cavalry corps had to be replaced by a new one.The French cavalry corps, exhausted and skeletal, were incapable of any overall action, even if one of their divisions managed to maneuver practically as far as the camp of Sissonne [1], demonstrating the persistence of this open interval until mid-September.

Lessons

Result of a tactical mistake by von Klück who had contravened Bülow's orders, the gaping space between the 1st and 2nd ^German armies - ^{at the} time it was de-drawn - was filled by the ^German army.The gap between the 1st and 2nd German armies - at the time it was called the gap or gap - had to be controlled in order to prevent the entire army of the right wing, Klück's 1st, from being enveloped. Unable to have this mission taken into account by either of the two armies concerned, and having no reserves for this purpose, Moltke resorted to a subterfuge by using an entire cavalry corps to mask it in the form of a curtain of cavalry deployed and retreating at the same rate as the entire left wing. In this way, Moltke was able to maintain the coherence of his overall scheme throughout the extremely critical phase of the retreat, allowing his recovery on the Aisne at which the front began to freeze.

1] See General Chambe's book: Adieu cavalry.

Saint Cyrien of the marshal class of Turenne, served in formations of the mechanized armoured corps before joining the 102nd class of the^{École Supérieure} de Guerre. Alternating assignments in general staff, troop corps and school. Head of the Operational Studies Office of the Land Logistics Force Command before joining the higher military education as an instructor. Currently a task officer at the Force Employment Doctrine Centre.

Associate Researcher at the Land Division of the Defence Historical Service, publishes military history articles in the CESAT Papers, Doctrine magazine and the NOR/Armoured Cavalry Bulletin.