While Joseph and Jourdan were haltingly mustering their forces in Leon, the allies began that series of rapid flanking movements on the north which decided the campaign.

Swinging forward his powerful left wing he manoeuvred the French out of one strong position after another.

The Tormes, the Esla, the Douro, the Carrion, the Pisuerga, none of these streams stopped his advance.
Joseph nowhere showed fight; he abandoned even the castle of Burgos, and, fearing to be cut off from France, retired behind the upper Ebro.
The official excuse given for this rapid retreat was the lack of provisions: but the diaries of two British officers, Tomkinson and Simmons, show that they found the country between the Esla and the Ebro for the most part well stocked and fertile.

Simmons, who was with the famous Light Division, notes that the Rifles did not fire a shot after breaking up their winter quarters, until they skirmished with the French in the hills near the source of the Ebro.

The French retreat was really necessary in order to bring the King's forces into touch with the corps of Generals Clausel and Foy, in Navarre and Biscay respectively.