In making war upon France the Austrian forces were obliged to advance along both the Danube and the Po; for if they confined their operations exclusively to the valley of the Danube, they must yield northern Italy to the French; or if they restricted their operations wholly to the valley of the Po, they must lose western Germany, and leave unguarded the direct route between France and Austria. The Austrian forces were, therefore, divided into two armies: one of which confined its operations to the valley of the Danube; the other, to the valley of the Po. In advancing towards France, these armies became farther and farther separated from each other. Kray's army in western Germany and the army of Melas in northwestern Italy were separated by Switzerland and the great chain of the Alps. As Switzerland and the passes of the Swiss Alps were held by the French, there could be no direct communication between Kray and Melas. Though the great highway of the Tyrol, which crossed the Alps over the Brenner Pass, was in possession of the Austrians, it was so far in rear of the Austrian armies that re-enforcements could not be sent over it from one army to the other without making a march of several hundred miles. In fact, the nature of the country was such that during active operations neither army could expect to receive any support from the other. They were independent armies of equal strength. Each had a separate commander, and each had its own line of operations and its own line of retreat. On the other hand, the three French armies were so situated that they could support one another. With Moreau's army on the Rhine, Masséna's along the Apennines, and the Army of Reserve between them near Lake Geneva, Bonaparte could move the last along the roads of eastern France to re-enforce either of the others as circumstances might require. In this way Bonaparte could re-enforce Masséna with the Army of Reserve, which would increase the Army of Italy to eighty thousand combatants. Or, should Masséna be driven back across the Var into France, Bonaparte could leave Moreau on the defensive along the Rhine with a part of his army, withdraw the remainder, unite it with the Army of Reserve, and with these combined forces added to the remnants of Masséna's army, destroy Melas in the south of France.