These twenty-four corps were divided into three armies—the Army of the Meuse, based on Cologne; the Army of the Moselle, based on Metz and Coblenz, and the Army of the Rhine, based on Strassburg. All of these three armies were naturally to converge on Paris. The route of the Army of the Meuse would pass through Liége, Namur, and Maubeuge, and would therefore have to cross a part of Belgium; the Army of the Moselle would take a route through Sedan and Soissons, passing north of the Verdun fortress, but of necessity crossing the Duchy of Luxemburg; the Army of the Rhine, after crossing the screen of the Vosges Mountains, would pass through Nancy and Toul, between the fortresses of Epinal and Belfort.
It is obvious that the march to Paris would be most quickly achieved through the flat country of Belgium, where the French frontier is practically unguarded and only the weakly manned barrier fortresses of Belgium barred the way. The remainder of the French frontier from Luxemburg to Switzerland was well fortified, and Germany had no time to spend in reducing fortified places.
The main advance was therefore to take place through Belgium, the Army of the Moselle co-operating, while to the Army of the Rhine was assigned the offensive-defensive rôle of advancing to the barrier fortresses of Epinal and Belfort to check any French advance that might be directed against the communications of the Armies of the Moselle and the Meuse to the north. The railroad communications through the Belgian plain were splendidly adapted to this plan, backed as they were by the military railroads which Germany had constructed several years before, running through the industrial districts in the north of the German Empire up to the Belgian border.
Germany's first move was the invasion of Luxemburg, violating the neutrality of a State which, under the treaty making her independent and guaranteeing neutrality, (to which treaty Germany was a party,) was not permitted to maintain an army. Two days later Germany asked passage for her troops through Belgium, for the purpose of attacking France. Belgium promptly refused, and on Aug. 4 Germany began the forcing of this passage by an attack on Liége.
Thus, at the outset the German plan went awry. Although the contemplated line of advance was through Liége and Namur, it was not sufficient, with Belgium openly in arms to defend her country, to reduce only these two towns. The Belgian Army could, and later did, fall back to the north on Louvain, Brussels, and Antwerp, and so be directly on the German flank and in a position to strike at the line of communications. It was therefore necessary to subjugate all of Belgium either by destroying the Belgian Army or driving it before them in their advance.
Thus, the German advance was not only doomed to delay, but at least 100,000 troops were needed to garrison a hostile country and to protect the life lines running to the rear.
Three days after the attack on Liége opened the Germans penetrated between the outer forts, their infantry advancing in close formation and sustaining enormous losses. But Liége was worth the price paid. Some of the forts held out for days, but were finally reduced by the fire of the 42-centimeter guns—the first of the German surprises. The Belgian garrison, however, had done its work. The German advance was delayed for ten precious days, during which the first consignment of the British expeditionary force had reached the Continent and France and Russia had largely completed their mobilization.
As soon as it was realized that the unexpected Belgian resistance had retarded the German advance and in all probability had disarranged the German plan of campaign, the French, even before the guns of Liége had cooled, struck at Alsace, through the Belford Gap and over the Vosges Mountains. At first this French offensive was successful. Points on the Metz-Strassburg Railroad were taken and the town of Mülhausen captured. But almost before the news of success reached Paris the French had been defeated, not only in Alsace but also in Lorraine, whence French troops had been sent to engage the German Army of the Moselle. The result was the retirement of the French to the line of their first defense—a line that had been prepared for just such an emergency during the years since 1871.
While the German armies of the Moselle and of the Rhine were thus occupied in repelling the French advance the Army of the Meuse was forcing its way through Belgium. Throwing out a strong cavalry screen in its front, this army advanced through Tongres, St. Frond, Laugen, Haelen, and Terlemont, and finally confronted the Belgians on the line from Louvain to Namur. Fighting on this front filled almost a week, when the destruction of the fortifications of Namur forced the Belgians to fall back, pivoting on Louvain to the line from Louvain to Wavre, the last line in front of Brussels. On Aug. 20 the Belgians were defeated at Louvain and the Germans entered Brussels, the Belgian Government having previously retired to Antwerp. The first phase of the German advance was thus completed and the way to France was open.