Portugal presents three sections of frontier to an invader coming from the side of Spain. The northernmost, that from the mouth of the Minho to Miranda-de-Douro, was not within the scope of operations in 1810. It can only be approached from Galicia; that province was not subdued, nor had the French any intention of dealing with it till after they should have dealt with Portugal. An invasion of the Tras-os-Montes and the Entre-Douro-e-Minho would have been an objectless operation: they would fall of themselves if once Lisbon were captured and the English expelled from the Peninsula. A move against Oporto by some flanking division of the invading army might have been conceivable, but such an attempt would be made, if made at all, from the south of the Douro, through northern Beira, and not through the mountains of the Tras-os-Montes. There remain two other sections of the Portuguese frontier: the one from the Douro to the Tagus, and the other from the Tagus to the Guadiana. Both of these were accessible to the French in 1810, since they were in possession alike of the plains of Leon and of La Mancha, and of northern Andalusia. It was open to them to choose one or the other front for attack, or to attack both at once. Lisbon being the objective, it was clear that an attack on the northern or Beira frontier possessed a paramount advantage over an attack on the southern or Alemtejo frontier. A successful advance north of the Tagus brings the invader directly to the gates of Lisbon; one south of the Tagus brings him only to the heights of Almada, where he is separated from the Portuguese capital by the broad estuary of the Tagus. Napoleon’s power, like that of the devil in mediaeval legends, ended at the edge of the salt water; and in face of the naval strength which the English always maintained at Lisbon, a victorious French army camped on the heights of Almada would be almost as far from final success as when it started from Spain. The 1,900 yards of strait which protected the Portuguese capital could not be crossed. The most that the invader could accomplish would be to worry the ships in the port, and the lower quarters of the city, by a distant bombardment, if he could bring up heavy guns from Spain[152]. For nearly twenty miles inland from Lisbon the estuary of the Tagus expands into a broad brackish lagoon four to eleven miles broad, a complete protection against any attack from the east. Only at Alhandra does this inland sea contract, and for some further miles northward from that point the eastern bank of the Tagus is formed by broad salt-marshes (_lezirias_) cut up by countless channels of water, and practically inaccessible. It is only at Salvaterra, thirty miles north of Lisbon, that the Tagus assumes its ordinary breadth, and becomes an ordinary military obstacle. From that point upwards an invader from the Andalusian side might endeavour to cross it, and it presents no more difficulties than any other broad river.