barbarism, and later still conquered them with the sword.  All through the succeeding centuries the ambitions of kings in France, or of emperors upon the Rhine, were checked or satisfied in that natural avenue of advance.  Charlemagne’s frontier palace and military centre facing the Pagans was rather at Aix than at Treves or Metz; and though the Irish missionaries, who brought letters and the arts and the customs of reasonable men to the Germans, worked rather from the south, the later forced conversion of the Saxons, which determined the entry of the German tribes as a whole into Christendom, was a stroke struck northwards from the Belgian Plain.  Caesar’s adventurous crossing of the Rhine was a northern crossing.  The Capetian monarchy was saved on its eastern front at Bouvines, in that same territory.  The Austro-Spanish advance came down from it, to be checked at St. Quentin.  Louis XIV.’s main struggle for power upon the marches of his kingdom concentrated here.  The first great check to it was Marlborough’s campaign upon the Meuse; the last battle was within sound of Mons, at Malplaquet.  The final decision, as it was hoped—­the defeat of Napoleon at Waterloo—­again showed what this territory meant in the military history of the West.  It was following upon this decision that Europe, in the great settlement, decided to curb the chaos of future war by solemnly neutralizing the Belgian Plain for ever; and to that pact a seal was set not only by the French and the British, but also by the Prussian Government, with what results we know.

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 Liege.  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 Mezieres 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 Liege 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 Liege, L, and so passed through Aix-la-Chapelle, A, and to Germany.