Just at the time when Wellington turned his back on Burgos, Napoleon's retreat from Moscow began. Before the end of the year he was driven out of Russia, having lost nearly half-a-million of men: Prussia also, hitherto his nominal ally, rose in arms against him. In order to make head against Russia and Prussia on the Elbe, he had to withdraw troops from Spain, besides directing to Germany reinforcements and supplies which otherwise might have been devoted to the Spanish armies. Hence when the campaign of 1813 began, Wellington had the superiority of strength, though his enemies were now less widely scattered than before the evacuation of Andalusia. There could be no doubt that to invade Spain once more in the same general direction as in 1812 would be the most effective. Even Sir John Moore's small force, boldly plunging into Spain by that route at the end of 1808, had made Napoleon fear for his line of communication with France, and compelled him to detach overwhelming forces against the English. _A fortiori_ Wellington, advancing by that line with an army equal to all that king Joseph could bring together, must compel him to evacuate Madrid, and retreat sufficiently far northwards to guard the main road to France. Wellington knew by his experience of the last year that the line of the Douro beyond Salamanca was difficult to force in the face of a fairly equal enemy. He therefore resolved that a large portion of his army should cross the Douro down in Portugal, and then move eastwards, while he himself advanced _viâ_ Salamanca. The pressure of his left wing would compel the French in his front to retire, for fear of being completely outflanked. And here came in the advantage which he derived from the English command of the sea. Instead of dragging behind him an ever-lengthening chain, in the shape of communication with Lisbon, which had hitherto been his base for supplies, he could, if confident of having the upper hand in the north-west of Spain, have his supplies brought to the northern ports, and conveyed thence by comparatively short journeys. And he was confident, so thoroughly so as to let it be seen; it is told that when he passed the frontier into Spain, he rose in his stirrups, and looking round waved his hand, exclaiming: "Farewell, Portugal!" During four weary years he had stood substantially on the defensive, guarding the frontier if he could, retreating if he could not, obliged to withdraw behind it after incursions into Spain. Now at last his turn was come, and he knew it. [Illustration: _Map XV._ Battle of Vittoria. 21^{st} June 1813.]