Lannes could not fail to see that if he committed himself to the final assault on Saragossa, and entangled the 3rd Corps in street-fighting, he might find himself assailed from the rear on all points of his lines. There were no troops whatever in front of Saragossa to form a ‘covering-force’ to beat off the insurgents, if they should come down upon his camps while he was storming the city, for the 3rd Corps and Gazan’s division had now only 20,000 infantry for the conduct of the siege. Accordingly the Marshal resolved to undo the Emperor’s arrangements for keeping up the line of communication with Madrid, and to draw in Mortier, with Suchet’s strong and intact division, from Calatayud, where he had been lying for the last three weeks. This was the only possible force which he could use to provide himself with a covering army. The touch with Madrid, a thing of comparatively minor importance, had to be sacrificed, except so far as it could be kept up by the division of Dessolles, which had now come back from the pursuit of Sir John Moore, and had pushed detachments back to its old posts at Sigüenza and Guadalajara. Mortier therefore evacuated Calatayud by the orders of Lannes, and came back to the Ebro: passing behind the besieging army he crossed the river and took post at Perdiguera with 10,000 men, facing the levies of Perena in the direction of Huesca. It was only when he had made certain of having this powerful reinforcement close at hand, ready to deal with any interference from without, that Lannes dared to proceed with the assault. At the same time that Mortier arrived at Perdiguera, he sent out Wathier, with two battalions and two regiments of cavalry, to deal with the insurgents of the Lower Ebro, where Francisco Palafox had been busy. Four or five thousand peasants with one newly-levied regiment of Aragonese volunteers tried to resist this small column, but were beaten on the twenty-sixth in front of the town of Alcañiz, which fell into Wathier’s hands, and with it 20,000 sheep and 1,500 sacks of flour, which had been collected for the revictualling of Saragossa, in case the investment should be broken. They were a welcome windfall to the besieging army, where food was none too plentiful, since the plain country where it lay encamped had now been eaten bare, and convoys of food from Tudela and Pampeluna were rare and inadequate.