The Germans will achieve a decisive success on the French in August or September, bringing the war to a relatively quick close. Terrence M. Holmes, writing in
"Not the Schlieffen Plan 1914", explains the situation thusly:
If Moltke had followed Schlieffen’s real intentions for the counter-offensive conduct of a two-front war, the first great battle of 1914 would have been fought in Lorraine in the third week of hostilities, on terms much more favourable to Germany than they were at the battle of the Marne. We can reconstruct this alternative scenario because we know exactly what the French chief of staff Joseph Joffre intended to do if the Germans did not invade Belgium.
French war planning was constrained by two political imperatives. In the first place, France was committed by agreement with her Russian ally to launch an ‘all-out and immediate’ attack against Germany as soon as possible after the outbreak of war. Moreover, the French government had resolved not to encroach on Belgian territory unless the Germans did so first. Joffre was therefore obliged to incorporate in his war plans a variant which allowed for a full-scale offensive avoiding Belgian territory altogether, and that would have come into effect in 1914 if the Germans had stayed on the defensive and not entered Belgium. For this eventuality Joffre decided that three of his five armies, comprising some 60 percent of his first-line troops, should invade Lorraine on 14 August, aiming initially to reach the line of the river Saar between Sarrebourg and Saarbrücken (Doughty 2010, 146-8, 155-8, 168). Ominously, that position was flanked at both ends by the German fortresses of Metz and Strasbourg.
Schlieffen had long before outlined how the Germans should exploit a massive French incursion through ‘the relatively narrow space between Metz and Strasbourg’. The aim must not be to push the enemy back to his fortified border. Rather, he had to be engaged on three sides, ‘from Metz, from the Saar and from Strasbourg’, and brought to a standstill there, which would give the Germans an excellent chance of
decisive victory by means of envelopment attacks out of Metz and Strasbourg. The ultimate aim of this ‘attack on the enemy’s flank and rear’ would be to surround the French invasion forces and ‘not just defeat them, but lay them low and as far as possible annihilate them’ (Boetticher 1933, 260).
Joffre himself was acutely aware of the perils attending a French offensive in Lorraine. He said that the object would be to rupture the German front, but he conceded
that:
"In the course of this operation our forces would be liable to be taken in flank by attacks coming in all probability from both Metz and the region of Molsheim-Strasbourg. By penetrating like a wedge into the midst of the enemy’s lines we would be more or less inviting envelopment (Joffre 1932, 74-5)."
But a German defensive posture in 1914 would have compelled Joffre to embark on that hazardous course of action — that was precisely what he was committed to if the Germans refrained from attacking through Belgium and waited instead for the opportunity to counter-attack. In that event, the war would have started with a great battle of encirclement as soon as the French First, Second and Fourth Armies had completed their short advance into the danger zone between Metz and Strasbourg. Speaking in 1904 of the strategic importance of these fortresses, Schlieffen once again emphasized their role in counter-offensive operations: ‘I do not mean a Metz and Strasbourg that are to be besieged and defended, but rather a Metz and Strasbourg in which armies are assembled and through which they march in order to attack the enemy by surprise’ (Zuber 2004, 160).
In short,
60% of the French Army in August of 1914 was walking into a trap, which they only escaped thanks to the French re-orienting themselves to respond to the German advance through Belgium and into France itself. It would've been a war winning blow from which the French never would've recovered from. An Eastern Front-first strategy will also allow the Germans to build up four armies-42 Divisions-in the East which will force the Russians onto a defensive stance and thus allow Austro-Hungary to avoid the Galicia disaster that so battered their Army IOTL 1914. Serbia thus is likely destroyed by 2nd Army in October, as OTL their severe munition shortages nearly forced the Serbs to surrender anyway.
