So, the German plan pre-war was for the 1st army to pass west of Paris and invest it, while 2nd army passed east of Paris, and invested that side. The difficulty of a direct assault on Paris are the historic fortifications that ring the city. In the 1870 war, the German army was able to beat the French army in the field, but then it took until 1871 for the Parisian government to surrender during a siege. Regardless of the value of those fortresses in 1914, that memory still dictated the plan for 1914.

So why did they change that plan? There are two reasons. The first is that the German army had too few men to man a front which stretched out around Paris. The combat along the border had been especially difficult, and the Germans required a heavier concentration then they had expected to defeat the French. Then wrap in the casualties they had sustained, and the 3 corps of troops which were transferred from west to east just prior to the battle, and you can start to the see the manpower problem faced by the German army. So, Molkte had 1st Army move east of Paris, and the 2nd army would make a similar shift to its left, to keep in contact with the 3rd Army. This was fine, because before the Marne it was believed (mostly correctly) that the Paris garrison was more of a defensive force than an offensive one. The French Army was out of place, and 1st and 2nd Armies were seriously threatening their flank, as well as the BEF's connection w/ the Channel. Unfortunately, the Paris Garrison was reinforced, and was commanded by a general who was very offensively minded. The Paris garrison launched its own flank attack, which nearly caught 1st Army in the flank. The battle of the Marne then, was the repositioning of 1st Army from a pass by, to a direct attack on Paris.

So, could the Germans have won if they held their own plan? Why did they lose on the Marne? Could they have won that battle? I think that the change from the original Schlieffen Plan to the modified plan made logical sense. The German Army simply didnt have the strength required to complete that move west of Paris. They had a hard enough time with their positioning during the Marne, so imagine doubling or even tripling the distance between each German Army, their supply lines would have likely been cut. This is especially true given the ferocity of the Parisian garrison. Thats not necessarily a death sentence for the 1st Army, but going much further brings us into some heavy counterfactual territory. So why did the Germans end up losing? As the Parisian Garrison came out of the city and pushed against 1st Army, Kluck's army became absorbed in defeating this counterattack. Because of this, a gap opened up between the 1st Army and the 2nd Army. If this gap was detected and exploited, the Entente forces could have pushed on both armies flanks, or maybe even cut the supply lines of the flank corps, and the whole army. The BEF just happened to be pushing (tentatively) towards this gap, which really worried von Bulow in the 2nd Army (who had been appointed commander of the right wing attack. Basically, he was in charge of both 1st and 2nd Army). Kluck thought that he and Bulow should just ignore the BEF and continue against Paris. Kluck believed that if he counterattacked the Parisian Garrison himself, he would have beaten it back and taken the city. But Bulow argued that he could not ignore such a threat, and that they armies had to pull back to defensible ground and reconnect the flanks. Bulow, being in charge, won out against Kluck. Was that the right decision? I think not. I believe Kluck would have succeeded in pushing in the Paris Garrison (at the least, he would have immobilized it in the city), and I think that the BEF wasnt yet a critical threat. If you look at its combat performance under Gen. French, the BEF could hardly have been called aggressive, or fast moving. Even during the Marne counterstroke, French only grudgingly moved the BEF back towards the Germans. I dont think they could have exploited the gap in time to save the Parisians, and I think that if 1st Army could have moved any troops into the gap, or against the BEF's flank (after halting the Parisians), French would have halted his movement, and likely fallen back. But this ventures back into that whole "counterfactual" area.

And it almost goes without saying that every historian Ive ever read on the topic assumes that had Paris fallen, France would have fallen. Paris has a weird position within France, it really is the political, economic, and cultural head of the state. Had it fallen (like in WW2), French resistance would have collapsed, I think, very soon after.

If youre looking for a book to read on the topic this centennial, I would recommend:

Holger Herwig, The Marne, 1914: The Opening of World War I and the Battle That Changed the World(New York: Random House, 2009).

Also, some people swear by Barbara Tuckman's The Guns of August. I have some problems with that book, and especially its interpretations of the failure of the Marne offensive. If you had infinite time and money, I think you would get something out of reading both. But if you have to pick one (and only one), Id really urge you to get Herwig's book.

Moltke's priority at this point was attempting to envelop a great deal of the French forces, namely the First, Second, Third, Fourth, Fifth, and Ninth Armies. The German Second, Third and Fifth Armies were to turn south early, instead of circling Paris, and drive the French forces in a south-easterly direction, away from Paris.

While Paris would have been nice to capture, and perhaps had a great effect on French morale/logistics/industry, it would also have involved defeating a fanatical (if not terribly well-prepared) defending force, and would have done little in the strictly military sense of taking large enemy forces out of the fight. Compared to encircling nine French armies, it was strictly a secondary objective.

Joffre had plans to establish another defensive line along the Seine, and leave Paris to the Germans if its capture looked likely. Although logistical and industrial problems would definitely have ensued for the French (Paris was a large node on their rail system, and the industrial capacity of the area was considerable) so early on in the war, the French were committed and took terrible losses and continued to fight. The morale effect may have been more pronounced on the civilian population.

Apologies if I've wandered off too far into what-ifs there for this sub.