The short answer, as I undrstand it, is that Joffre premised Fifth Army's advance on a prompt German violation of Belgium, which he thought virtually certain. It would solve his problem (of Belgian neutrality) for him.
There were three possibilities Plan XVII considered:
1) that the Germans would respect Belgian and Luxembourgeois neutrality and attack the Belfort–Épinal and Toul–Verdun lines; or
2) advance through Luxembourg in the vicinity of Verdun, then make a smaller attack into Belgium; or
3) defend in Lorraine and attack through Belgium (effectively some variant of what we know as the Schleiffen Plan).
Joffre banked on #3 (with no frontline use of German reserve corps!), with a possibility of #2, which of course would also serve his purposes.
Note what possibility is not considered:
Germany remaining on defense in the West.
Or was it?
I just came across a
previous discussion of this in an older thread, where
@AJE pulled up a paper by Terence Holmes at a conference six years ago, "
Not the Schlieffen Plan 1914":
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).
As AJE puts it: This may have failed if the French didn't use such a strategy, and they nearly did when Victor-Constant Michel, the de facto French Chief of Staff, made a defensive plan to counter a potential German move through Belgium. But the French generals and government rejected his plans due to a lack of offensive spirit, fired him, and replaced him with Joseph Joffre, who once again made offensive plans of the type that Schlieffen could take advantage of, and these were the plans that Holmes is describing, so it would have ultimately worked in that respect in 1914.
It explains why the Germans fortified Metz and Strasbourg to a very high degree while leaving the border between them, and therefore the iron ore mines, almost undefended. The same thing that made those mines an apparent vulnerability also made them useful as bait for the French to invade.
As to why Moltke diverged from this plan and created what is erroneously called the "Schlieffen Plan," the obvious and simple answer is that he was a fool. But more specifically, Holmes has this to say (at the very end of the article):
One obvious question remains: Moltke had studied the Schlieffen plan, so why did he pay no heed to Schlieffen’s argument about the number of troops that would be necessary for a decisive attack on France? The answer may perhaps be found in their opposing views of the relation between attack and defence. Echoing Clausewitz once again, Schlieffen maintained that ‘the defensive is the stronger form of war’, but Moltke was convinced that ‘the stronger form of combat lies in the offensive’ because it represents a ‘striving after positive goals’. He allowed that the offensive spirit could be blunted in a long-drawn-out assault on the French border position, but he thought that an attack ‘in the open’, brought about by an advance through Belgium, would lend the German army ‘the impetus and initiative that we need all the more, the greater the number of enemies we have to contend with’. Moltke subscribed to a then fashionable belief that the moral advantage of the offensive could make up for a lack of numbers. Unfortunately for the Germans, it was Schlieffen’s Clausewitzian outlook that was vindicated at the battle of the Marne.
