View attachment 821827
Taken from Wikipedia. More detailed sources are available.
Firstly, logistics. I know, logistics is boring and all the rest. The main Entente logistic hub is circled in blue, and - as we can see - it is very close to the French and British forces. Moving supplies - bullets and artillery shells and so forth - from the hub to the troops is comparatively simple.
In the German case, however, those logistical hubs are way, way back from the 1st to 5th armies, and those same supplies have to be moved from these hubs some 250 miles to the German troops at the sharp end, a lot of it through Belgium and northern France. Germany, at this time, was executing civilians in order to quell francs tireurs, who were - according to the Germans - responsible for disruption of these supplies.
Now, either there were such francs tireurs and hence there was disruption to the lines of supply, and the Germans had a reason (of sorts) for committing war crimes (and regarded as such by the standards of the time), or there wasn't such actions, and the executions were simply war crimes being committed because there was no-one to stop such activity.
Myself, I rather think it was a little from column A, a little from column B, but it is a very simple choice. Either there was disruption to the supply lines or the German forces were behaving as proto-Nazis, or both.
Now, we see the position on 9th September, just at the start of the race to the sea. Between the German First Army and the German Second Army is a gap. A very large gap, some 40 miles wide, in which only scattered detachments were positioned. Right in front of that gap is the BEF and the French 5th Army, with nothing much in front of them.
A move forward by these two, and the German 2nd to 5th Armies have no more ability to race to the sea than they have to race to the Moon. They are in danger of being flanked, with the very real prospect of having Entente forces breaking through to the rear areas. Most military strategists would regard this as a Bad Thing for the German forces.
Meanwhile, the German First Army is isolated. It has both flanks hanging, there's a French Army in front of it, and two French armies being gathered in Paris and in a position to get around.
Anyone who can look at that map and say that the Germans have the advantage in the Race to the Sea is - misguided.