But, though even Rhines and Danubes may be passed in the face of an enemy, the operation is not one which a prudent general courts, and the Tagus is broad, absolutely bridgeless, and fickle in the extreme in its alternations of high and low water. To fight one’s way from the valley of the Guadiana in order to meet such a problem at the end does not seem inviting. And even if the Tagus is passed, there are still thirty miles of road, including some formidable defensive positions, between the invader and Lisbon[153]. Yet there was one contingency under which an advance on the left bank of the river might be advantageous to the invader, and so possible was this contingency that Wellington from the very first had declared that he thought it probable that the French would move troops in that direction. If the Anglo-Portuguese army were drawn away to the Beira frontier, between Tagus and Douro, in order to resist a front attack delivered from the plains of Leon, and if it became involved in an active campaign somewhere far to the north, on the line of the Coa, or the Mondego, or the Alva, a subsidiary French force, striking south of the Tagus from the direction of Spanish Estremadura, might give dreadful trouble. If it could cross the Tagus anywhere between Abrantes and Salvaterra, it might get between the Anglo-Portuguese army and its base, and either fall upon its rear or capture Lisbon. For this reason Wellington, so far back as October 1809, had made up his mind that, if the French had an army on foot anywhere in the direction of Badajoz and Elvas, he must leave a considerable proportion of his own forces to watch them, and to defend, if need be, the line of the lower Tagus[154]. As long as the enemy had not yet subdued Badajoz and the neighbouring fortresses, and while there was still a strong Spanish army in that quarter, the need for precaution was not so pressing. Nevertheless, all through the summer of 1810 Wellington kept Hill with one English and one Portuguese division at Portalegre, south of the Tagus, though he withdrew this detachment when Masséna marched on Coimbra. Matters were much more perilous after the battle of the Gebora and the fall of Badajoz in February 1811. From that time onward, all through 1811 and 1812, nearly a third of the Anglo-Portuguese army was kept in the Alemtejo, first under Beresford, then under Hill, in order to guard against the possible stab in the back from the French army of Andalusia. [152] See Map of the Lines of Torres Vedras, in Section xx of this volume, for the environs of Lisbon. [153] Eliot, in his very judicious remarks on p. 100 of his _Defence of Portugal_, published just before Masséna’s invasion, sums up the situation with--‘a passage may without any difficulty be forced to the left bank of the Tagus: but then the enemy is as far from the accomplishment of his project as before, the river forming an insuperable barrier if well defended.’