The most important portion of this extensive line of frontier was undoubtedly that which fronted the Netherlands; for although it had been planned by the Allies that no advance was to be made by the troops in Belgium until the remainder of their forces had reached a line of connecting points along the French frontier, when all their Armies were to march, in combined movement, upon the capital: still it was reasonably to be expected that NAPOLEON would not wait for the completion of this plan, but rather that he would endeavour, by a decisive effort, if not to frustrate its accomplishment, at least to diminish its efficacy. It required no great exercise of military sagacity or political foresight to predict, that after having adopted a maturely considered disposition of force on the most important points along his general line of defence, and placed his frontier Fortresses upon a respectable footing, NAPOLEON would open the tremendous game, upon which his crown, his political existence, and the fate of France, were now fairly staked, by a bold, sudden, and resolute advance into Belgium--straining every nerve to vanquish, in detail, the Allied forces in that densely populated country; of which a vast portion was already prepared to declare in his favour. His authority once established in Brussels, through the means of some great and signal triumph, the accession to his moral influence over the entire mass of the French nation would be immense; and then, flying to the succour of his nearest Corps menaced from the banks of the Rhine by the approach of hostile forces (upon which his possession of Belgium would operate as a powerful check by the facilities thus afforded for a combined attack in front and flank), a series of brilliant successes, supported by fresh levies from the interior, might enable him even to dictate terms to the Allies, who had indignantly rejected all his overtures. Hence the importance of narrowly watching the Belgian frontier, and of making due preparations for meeting any attack in that quarter, was too obvious not to form a principal feature in the general plan of the Allies. Its defence was assigned to an Army under the Duke of WELLINGTON, comprising contingent forces from Great Britain, from Hanover, the Netherlands, Brunswick, and Nassau; and to a Prussian Army, under Field Marshal Prince BLÜCHER VON WAHLSTADT.