The ideal line of invasion of France by Germany obviously lies through Belgium. It would turn and negative the chain of French fortresses on the eastern frontier, and give the shortest route through hostile territory to the French capital. Belgium is neutral ground; her neutrality guaranteed by the Great Powers; but how vain a pretence is this guarantee is already proved by the latest news from our Berlin Correspondent. It is believed (he states on credible authority), that Germany has been successful in exacting or obtaining from Belgium a secret Convention, whereby the armies of the Empire shall be free to traverse the former State, and to utilise for their purposes the Belgian railway system. The advantages of this arrangement may be said to fairly compensate Germany for the numerical superiority of the French forces over those which she herself is able to bring into the field. The German plan of campaign, as explained by our Berlin Correspondent, is as follows:—Seven of the twenty Army Corps are engaged on the Russian frontier under the King of Saxony. To cope with France there remain thirteen corps, with a proportionate number of independent cavalry brigades. The First Army, under the command of Prince Albrecht of Prussia, is to advance through Belgium by Verviers, Liége, Namur, and Charleroi, and cross the northern frontier of France between Maubeuge and Rocroy, at and about Hirson. The fortresses on the French northern frontier east of Maubeuge are of little account, and there are none on the section specified. The Ardennes and Eifel districts are regarded as affording considerable protection to the line of communication as far as the frontier, and a further protection will presently be mentioned. It is unfortunate that between Aix and Liége there is available but one line of railway, but the accommodation is copious on either side of this section, several lines being serviceable right to the frontier. This First Army is to consist of six Army Corps, the Guards, 7th, 8th, 10th, 11th, and 16th being those whose respective provinces are nearest to the region of concentration west of Cologne. Among its Corps-commanders are such men as Generals Meerscheidt-Hullessem, Von der Burg, Von Versen, Albedyll, Von Loë, all distinguished names in the war of 1870–71. The Emperor himself, who of course is Commander-in-Chief of all the German forces in both fields of operations, accompanies this army, after leaving the eastern frontier with General von Schlieffen, the Chief of the great General Staff, and a number of the German princes. The cavalry commander is General the Grand Duke Frederick of Baden.