The plan to attack through Belgium was obvious??

Was it?

Now IF the details of the Schlieffen Plan had been known to the French in 1904, 1905 or even 1906, then the French must have been rather mystified by what the Germans eventually ended up doing in the late summer of 1914 wouldn't they?

Now IF the details of the Schlieffen Plan had been known to the French in 1904, 1905 or even 1906, then the disposition of the French Armies would have been drastically different to those confronted by the Germans in the late summer of 1914 wouldn't they?

WHY??

Because Moltke changed and modified the Schlieffen Plan, and those modifications severely constrained the attack and reduced its chances of success. The Schlieffen Plan as envisaged by Schlieffen in it's original form looked more like the German attack in the West of 1940.

Schlieffen's dying words apparently were, "Remember keep the right strong." i.e. mass your strength on the right flank of the attack. Schlieffen looked at attacking France through both Holland and Belgium.

Schlieffen's original Plan required ALL of Germany's Army to execute the attack. The "eastern front" according to his thinking could be held initially by the Austro-Hungarians because of the time it would take Russia to mobilise it's forces. Moltke was not of the same opinion and took troops originally assigned to the German right flank and stationed them in the East - The Russian mobilisation eventually turned out to be far quicker than anticipated.

Schlieffen wanted to go through both Holland and Belgium because only going through Belgium would cause a bottleneck for the attacking force and severely limit the flow of supplies and reinforcements to the front (Take a look at the limited front afforded by the German Belgian Border). Railway links were far better between Germany and Holland and far better "North-South" between France and Holland in Belgium the closer you got to the coast (Paris-Brussels-Antwerp-Rotterdam). By sweeping down the coast it would effectively cut off the shortest line of communication between France and Great Britain forcing the British to concentrate a disproportionate number of ships from the Home Fleet in the southern part of the North Sea in a defensive posture thus weakening any blockade of Germany.

In the event the German right was not strong enough, the Belgians resisted and suffered the terror the Germans unleashed against the civilian population in their path, but they delayed the advance and further constricted the flow of supplies (Vital to an attacking army on the move). In meeting the six divisions of the BEF the German Army met the only force in the world at the time that were fully capable of fighting a withdrawal against overwhelming odds ("Fire-and Manouevre" tactics work equally well in both attack and retreat) in such a manner as to remain intact as an effective fighting force whilst inflicting serious casualties on that attacking force - again the German ability to bring up reinforcements delayed their progress and if anything the Schlieffen Plan required speed to be successful.

Delayed at both Mons and Le Cateau they were finally stopped at the Marne.