If Bonaparte had crossed the Po, he could undoubtedly have defeated Melas and driven him back to Alessandria; but here the Austrian commander, protected by the fortifications of the city, would doubtless have made a stand, and would have collected a large force to oppose Bonaparte. In this position, Melas would be joined by General Elsnitz, already marching on Alessandria, and could receive re-enforcements from General Ott and from General Vukassovich. Thus, in a short time, he could outnumber Bonaparte's army. Moreover, at Alessandria he could prevent the junction of Moncey's corps with the Army of Reserve, and might possibly be able to defeat them in detail. In this position, Bonaparte, if defeated, would have no unobstructed line of retreat, for Fort Bard was still held by the Austrians; and, if victorious, he could do no more than force Melas back along the great highway to Mantua. In fact, should Melas be defeated at Alessandria, he could fall back to the fortress of Tortona or to the Stradella Pass, and there occupy another strong position. Here, with the re-enforcements that would doubtless join him from Genoa, from Vukassovich's corps, and from the fortresses in his rear, he would still have great chances of success. In short, it would have been the height of folly for Bonaparte, with no secure line of retreat, to march into the centre of the Austrian theatre of operations, and expect to conquer Italy with but thirty-five thousand men. Even though it had been necessary to sacrifice Masséna, Bonaparte would have been justified in marching on Milan; for, in no other way could he be joined by Moncey's corps; in no other way could he sever the Austrian communications, and in no other way could he hope to defeat Melas and conquer Italy. Victory was his object. The most critical part of Bonaparte's operations was the flank march from Ivrea to Milan; for at this time his only line of retreat was by way of the Great St. Bernard; and even on this route the Austrians still held Fort Bard. Under ordinary circumstances, a flank march is always more or less a hazardous undertaking. When a commander makes this movement and is attacked in route, he must form front to a flank,[10] and fight with his battle-line parallel to his communications, while the enemy can fight with his front perpendicular to his communications. In this position the advantage of the enemy is enormous. If victorious, he severs the communications of his adversary, and may then capture or destroy his army; if defeated, he can retreat in safety along his communications, or fall back to a new position, fight again, and thus prolong the conflict. An army without communications is like a rudderless ship adrift on the ocean. In order to fight, soldiers must have food and ammunition. No greater calamity, short of defeat, can befall a commander than to be cut from his base of operations and lose his source of supply.