On the morning of 19 February, French forces under Marshal Édouard Mortier quickly defeated the Spanish army, inflicting 1,000 casualties and taking 4,000 prisoners while losing only 400 men.

[5] Thirty thousand Allied troops and six major fortresses now stood between the French army and the Portuguese capital, making an attack against Lisbon virtually impossible.

[4] Nevertheless, compelled to act, Soult instead gathered an army of 20,000 men, mainly from V Corps, and launched an expedition into Extremadura with the aim of capturing the Spanish fortress at Badajoz, thereby drawing some of the Allied forces away from Masséna and the Lines of Torres Vedras.

[12] Soult was now in a difficult position: although he had a large (4,000-strong) contingent of cavalry, deploying two battalions to escort the prisoners taken at Olivenza back to French-held Seville left him only 5,500 infantry with which to continue his campaign.

Despite these problems, Soult decided to besiege Badajoz in hopes that Wellington would send reinforcements to the Spanish fortress and thereby reduce the Allied forces facing Masséna at the Lines of Torres Vedras.

[16] Weakened by the defeat at Olivenza and by Ballesteros' continued absence, he sent to La Romana for reinforcements, receiving on 14 January 1,800 men sent from Abrantes under the command of Carlos de España.

When these forces joined with Mendizábal's remaining 3,000 men, a Spanish cavalry division and a brigade of Portuguese horse, the Allies had an army almost 15,000 strong—intended to be under the command of La Romana—with which to hold Soult in check.

The Spanish commander again ignored Wellington's plan, failing to dig entrenchments on the heights, nor did he send out a cavalry screen to protect his front and monitor the French movements.

[24] Heavy rains also flooded both the Guadiana and Gebora rivers, rendering them impassable, so that between 11–18 February the French were only able to shell the southern end of the Spanish line, pushing the Spaniards further away from Badajoz and the protection of the San Cristóbal fort.

[25] That evening Soult sent nine infantry battalions, three cavalry squadrons and two artillery batteries, under Mortier's command, to the north bank across a flying bridge over the Guadiana River.

[26] Due to heavy fog that morning, Mendizábal was unaware of the approaching French until his picket, only a mile from his front, was driven back by Mortier's infantry fording the Gebora.

[27] At the same time the 2nd Hussars, sent by Latour-Maubourg to turn the Spanish left flank, had managed to climb the heights to the north, also undetected, and fell upon one of de España's unsuspecting regiments.

[31] The musketry duel between the two sides had scarcely begun, however, when the French cavalry appeared; the light horse approached along the top of the heights while Latour-Maubourg's dragoons advanced from the rear.

[33] The battle was a serious setback for the Anglo-Spanish-Portuguese allies; Wellington had earlier warned the Spanish generals that the Army of Extremadura was "the last body of troops which their country possesses",[34] and later wrote that "[t]he defeat of Mendizábal is the greatest misfortune, which was not previously expected, that has yet occurred to us.

[36] Soult was now free to continue his investment of Badajoz; although the town's garrison was now some 8,000 strong due to the influx of soldiers from Mendizábal's destroyed army, it eventually fell to the French on 11 March.