88 The Great War would take them into Germany and between the for- tresses of Strassburg and Mayence. As an alterna- tive, they were if successful free to turn west, isolate Metz and attack the flank and rear of the German armies invading France by moving to the Moselle Valley north of Metz and Thionville. Certain advantages, evidently, the French offensive had over the German. First, it began in French ter- ritory and crossed directly into German. The Ger- mans on the contrary had to cross Belgium, to es- tablish their communications in a hostile country and to fight a Belgian army before they could reach French territory, and they had not yet reached it. The population of Alsace-Lorraine, the field of the first phase of French operations, was friendly to France and could be relied upon to give all possible help to the invaders. The main line of communica- tion of the Germans must necessarily be across hos- tile Belgium for a hundred miles, and would require a strong rear guard. France could reach the Rhine and the Alsace-Lorraine frontier of the Bavarian Palatinate and Baden before she need consider her communications. The chief justification of the taking of Alsace- Lorraine in 1 871 has always been asserted by Ger- mans to be found in the fact that the presence of the French on the Rhine left Germany open to attack. While Strassburg and Metz could doubtless have sustained long sieges before they could be captured, a French ofi^ensive which isolated them and gave