The time limit, however, imposed certain other conditions. The Franco-German frontier from Luxemburg to Switzerland had been transformed into one long barrier, garnished with detached forts and resting upon the first-class fortresses of Verdun, Toul, Epinal, and Belfort. To pierce such a barrier was not impossible but to break through in three weeks, with the whole French army before the forts and the shortness of the front offering the Germans no opportunity to take advantage of their superior numbers, was recognized as next to impossible. There was left the roads through Belgium and Luxemburg. To come this way Germany had for more than a decade been constructing strategic railways leading from the Rhine and Moselle valleys to the Belgian frontier, double-track roads that served in a desolate country, but were provided with all the necessary machinery for detraining thousands of soldiers. Belgium might not consent to suffer this invasion of her territory but the Belgian army was negligible, and the German heavy artillery was known to be adequate to dispose of the antiquated forts of Namur and Liege with brief delay. Once the Germans had passed the Meuse and deployed upon the Belgian plain, they could turn south and pass the Franco-Belgian frontier, which was destitute of real defenses, the few fortresses being obsolete, and thence the road ran down to Paris clear and open. Conceivably Great Britain might make the Belgian invasion a cause for joining France. But, again, the British army was small, there was the gravest doubt as to whether it would be sent to the Continent at all, and even if it came, it would not redress the balance between the French and German armies. Such being the case, as German high command saw it, Belgium was summoned, and refusing, was attacked, the German armies passing the Belgian frontier in the direction of Liege on August 4, 1914, the day on which Germany declared war upon France, and the forty-fourth anniversary of the invasion of France in the Franco-Prussian War. FROM THE MEUSE TO THE MARNE To grasp the main circumstances of the opening campaign it is simplest to think of the whole German invading forces as comprising one army. The right of this army under Kluck and Buelow came west through Belgium by Brussels and Namur, swinging south after the Belgians were disposed of, and leaving a guard to curtain the Belgian army which had retreated on Antwerp. The center moved southwest through the Belgian Ardennes and Luxemburg, entering France between Longwy and Givet on the Meuse. The left moved from Metz and Strassburg, attempting to force the French barrier line between Toul and Epinal. The center was commanded by the German Crown Prince, Albert of Wuerttemberg, and Hausen, the left by the Crown Prince of Bavaria and Heeringen. Smaller forces operating in Upper Alsace played little real part in the operations.