The entries into this plain are very clearly defined by natural limits. It is barred a few hours' march beyond the German frontier by the broad and deep river Meuse, which here runs from the rough and difficult Ardennes country up to the Dutch frontier. The whole passage is no more than twelve miles across, and at the corner of it, where the Meuse bends, is the fortress of Liége. West of this fortress the upper reaches of the river run, roughly east and west upon Namur, and after Namur turn south again, passing through a very deep ravine that extends roughly from the French town of Mezières to Namur through the Ardennes country. The Belgian Plain is therefore like a bottle with a narrow neck, a bottle defined by the Dutch frontier and the Middle Meuse on either side, and a neck extending only from the Ardennes country to the Dutch frontier, with the fortress of Liége barring the way. Now the main blow was to be delivered ultimately upon the line Namur-Charleroi-Mons. That is, the situation was roughly that of the accompanying diagram: by the bottle neck at D the whole mass of troops must pass--or most of them--which are later to strike on the front AB. To reach that front was available to the invader the vast network of Belgian railways RRR crammed with rolling stock, and provided such opportunities for rapid advance as no other district in Europe could show. But all this system converged upon the main line which ran through the ring of forts round Liége, L, and so passed through Aix-la-Chapelle, A, and to Germany. [Illustration: Sketch 33.] The German Government, therefore, could not be secure of its intention to pass great bodies through the Belgian Plain until Liége was grasped, and it was determined to grasp Liége long before the mobilization of the German forces was completed. For this purpose only a comparatively small force, rapidly gathered, was available. It was placed under the command of General von Emmerich, and its first bodies exchanged shots with the Belgian outposts early in the afternoon of Tuesday, August 4, 1914. The hour and date should always be remembered for the solemnity which attaches to the beginning of any great thing; and the full observer of European affairs, who understands what part religion or superstition plays in the story of Europe, will note this enormously significant detail. The first Germans to cross the violated frontier accomplished that act upon the same day and at the same hour as that in which their forerunners had crossed the French frontier forty-four years before. The afternoon wore on to night, with no more than a conflict between outposts. Just before midnight the cannonade was first heard. It also was the moment in which the ultimatum delivered to Germany by this country, by a coincidence, expired.[2] This night attack with guns was only delivered against one sector of the Liége forts, and only with field-pieces.