[154] Wellington to Col. Fletcher, commanding Royal Engineers, Oct. 20, 1809 (_Dispatches_, v. 235): ‘The enemy will probably attack on two distinct lines, the one south, the other north of the Tagus, and the system of defence must be founded upon this general basis.... His object will be, by means of the corps south of the Tagus, to turn the positions which we shall take up in front of the corps north of that river, to cut off from Lisbon the corps opposed to him, and to destroy it by an attack in front and rear. This can be avoided only by the retreat of the right, centre, and left of the allies to a point at which (from the state of the river) they cannot be turned, by the passage of the Tagus by the enemy’s left corps.’ Six days later (_Disp._ v. 245) Wellington wrote to Admiral Berkeley in similar terms: ‘It is probable that in the event of the enemy being enabled to invade this country in force, he will make his main attack by the right of the Tagus: but he will employ one corps on the left of the river, with the object of embarrassing, if not of preventing, the embarcation of the British army.’ But the attack south of the Tagus was in Wellington’s, and also, we may add, in Napoleon’s conception[155], only a secondary operation. The main invasion was almost inevitably bound to take place on the Beira, not on the Alemtejo frontier. Between Fregeneda, where the Portuguese border line quits the Douro, to the pass of Villa Velha on the Tagus there is a distance of somewhat more than 100 miles. The division between Portugal and Spain does not lie along any well-marked natural feature, such as a mountain range or a broad river--though two small sections of the frontier are coincident with the insignificant streams of the Elga and the Agueda. It is rather drawn, in a somewhat arbitrary and haphazard fashion, through the midst of the desert upland, where Spain and Portugal turn their backs to each other. For the only piece of flat plain-land on the whole border is that from the Douro to Almeida, a mere ten or twelve miles, and immediately behind the Coa, only three or four miles from Almeida, the mountains begin. The rest of the frontier runs through thinly-peopled, barren highlands, from which the Coa, the Mondego, the Zezere, and the Ponçul fall away towards Portugal, and the Agueda and the Alagon towards Spain. The mountains are not, for the most part, very high--the culminating peak of the Serra da Estrella is only 6,540 feet--but they are singularly rugged and scarped, and much cleft by ravines, along whose sides the few roads crawl miserably, in constant precipitous dips and rises. This broad belt of upland, one long series of defiles for an invader, is some hundred miles broad, and does not cease till Coimbra, on the one side, or Abrantes, on the other, is reached: only then does the plain-land begin, and the country-side become fertile and thickly peopled.