Several lines of operation were open to the prince of Esling. 1º. He could pass the Tagus, between Punhete and Abrantes, by boats or by fords, which were always practicable after a week of dry weather. 2º. He could retire, by the Sobreira Formosa, upon Castello Branco, and open a communication with the king by Placentia, and with the duke of Dalmatia by Alcantara. 3º. He could march, by the Estrada Nova and Belmonte, to Sabugal, and afterwards act according to circumstances. 4º. He could gain the Mondego, and ascend the left bank of that river towards Guarda and Almeida; or, crossing it, march upon Oporto through an untouched country. Of these four plans, the first was perilous, and the weather too unsettled to be sure of the fords. The second and third were difficult, from the ruggedness of the Sobreira, and exposed, because the allies could break out by Abrantes upon the flank of the army while in retreat. Massena decided on the last, but his actual position being to the left of the line of retreat, he was necessarily forced to make a flank movement, with more than ten thousand sick men and all his stores, under the beard of an adversary before he could begin his retreat. Yet this he executed, and in a manner bespeaking the great commander. Commencing his preparations by destroying munition, and all guns that could not be horsed, he passed his sick and baggage, by degrees, upon Thomar, keeping only his fighting-men in the front, and at the same time indicating an intention of passing the Zezere. But when the impediments of the army had gained two marches, Ney suddenly assembled the sixth corps and the cavalry on the Lys, near Leiria, as if with the intention of advancing against Torres Vedras, a movement that necessarily kept lord Wellington in suspense. Meanwhile, the second and eighth corps, quitting Santarem, Tremes, and Alcanhete, in the night of the 5th, fell back, by Pernes, upon Torres Novas and Thomar, destroying the bridges on the Alviella behind them. The next morning the boats were burnt at Punhete, and Loison retreated by the road of Espinal to cover the flank of the main line of retreat; the remainder of the army, by rapid concentric marches, made for a position in front of Pombal: the line of movement to the Mondego was thus secured, and four days gained; for lord Wellington, although aware that a retreat was in execution, was quite unable to take any decided step, lest he should open the Lines to his adversary. Nevertheless he had caused Beresford to close to his right on the 5th, and at daylight, on the 6th, discovering the empty camps of Santarem, followed the enemy closely with his own army.