In 1810 Napoleon gave the chief command to Massena, the ablest of his marshals with one possible exception, the only one who had gained distinction at the head of an independent army. Massena's instructions were, in one of Napoleon's pet phrases, to drive the English into the sea, and the emperor, who could estimate no forces that could not be expressed in battalions, had every reason to expect that he would achieve his task. Wellington's scheme of defence was based on the geographical conditions. Three important rivers flow out of Spain across Portugal into the Atlantic. Where the Guadiana crosses the frontier stands the great fortress of Badajos on the Spanish side, faced by Elvas on the Portuguese. As both were at this time in the hands of the allies (Elvas indeed remained so throughout) they formed a serious obstacle to an attack on Portugal from this direction. Moreover it was obviously absurd for Massena to base his attack on the south, lengthening his communications by hundreds of miles: all that was possible was a subsidiary attack from the French army in Andalusia, already fully occupied with the hopeless siege of Cadiz. The Tagus valley is narrow, and barren of supplies, and almost as circuitous a route from France as the Guadiana. North of the Tagus, the Sierra de Estrella, which is a sort of continuation of the Sierra de Guadarrama, though at an angle to it, lies behind the frontier between Spain and Portugal. The roads across it into the Tagus basin were few and bad, and Wellington took care to render them worse. Thus the only route that needed serious defence was the northern one by the broad valley of the Douro, the natural and obvious course for an invader based on France, the easiest for an army, though not the shortest, between Lisbon and Madrid. Here also two fortresses faced each other across the frontier, Ciudad Rodrigo in Spain on the Agueda, Almeida in Portugal on the Coa, both rivers tributaries to the Douro. Strong as Massena was, he could not spare troops effectually to blockade these, and yet have sufficient superiority to drive Wellington before him. As a necessary preliminary therefore the two fortresses must be taken. Wellington took up his position near the frontier, so as to harass[69] Massena wherever it was possible without fighting a battle, and waited. Ciudad Rodrigo, weakly garrisoned by Spaniards, cost Massena forty days; the commandant, who did his best manfully, was naturally loud in his appeals for help, but Wellington turned a deaf ear. It would have been quixotic to fight a great battle against heavy odds to save a small garrison; as well might a chess-player sacrifice his queen to save a pawn. The turn of Almeida followed, though in consequence of an accidental explosion the siege did not last long. The way was now open for Massena to invade Portugal, though, thanks to Ciudad Rodrigo and to Craufurd, September had been reached. He resolved, as Wellington had hoped, to follow the course of the Mondego.