The combat of Castrillo (18 July 1812) was the second of two combats on the same day, and came after Marmont outmanoeuvred Wellington on the Douro and briefly threatened to cut off his rearguard.
In June Wellington launched an invasion of Spain, heading for Salamanca. Marmont ordered his main army to concentrate to the north of the city, but left small garrisons in three forts inside the city. Wellington detached part of his army to besiege the Salamanca Forts (17-26 June 1812), while the main army watched Marmont. The two armies almost clashed at San Cristobal (20-22 June 1812), but after the forts surrendered Marmont retreated to the Douro.
The two armies soon ended up watching a long stretch of the river. The French line was longer, running from Toros in the west to Simancas in the east. Wellington's line ran from the junction of the Trabancos and the Douro, at the fords of Pollos, to Tordesillas.
Marmont decided to try and convince Wellington that he was going to cross the river at Toros and head directly for Salamanca, and then turn back east to cross at Tordesillas, and get into Wellington's rear. On 15 July Foy and Bonnet, on the French right, were ordered to cross the river at Toros, while the main part of the army moved west along the north bank of the river.
The French movement was well underway by 16 July. Wellington fell for the ruse and ordered his army to move west. The 1st, 5th, 6th and 7th Divisions were ordered to move to Canizal and Fuente la Pena, west of the Guarena River. The 3rd Division, Bradford's Portuguese infantry and Carlos de Espana's Spanish infantry were to move to Castrillo on the Guarena, placing them just to the east of the centre of the new front line. The 4th and Light Divisions and Anson's cavalry were to form a rear guard, pausing at Castrejon on the Trabancos River, ten miles to the east of the main army.
Wellington discovered that he had been tricked during 17 July, but wasn't entirely sure what the situation was until late in the day. He then decided to lead two heavy cavalry brigades and the 5th Division east to help his rearguard withdraw, and arrived at the new front line early on 18 July. By this point the rearguard, under Stapleton Cotton, was already engaged with the advancing flight (combat of Castrejon, 18 July 1812). Wellington ordered the rearguard to retreat west to the Guarena, and despite the best efforts of the French, his troops reached the Guarena without suffering too many losses. The French then appeared on the heights east of the river, and Wellington ordered his troops to join the rest of the army. The 4th Division took up a position on the left, at the village of Castrillo. The Light and 5th Divisions joined the centre of the line. The 1st and 7th Divisions moved south to El Olmo. Wellington was now ready to defend against any French attack, and must have been hoping that Marmont would get carried away and try and attack the new British front line.
The French were pursuing in two columns. General Clausel commanded on the right, and it has his been troops that had posed the greatest threat during the retreat. He now decided to try and take advantage of the hurried Allied movement and attack the 4th Division, newly arrived on the heights above Castrillo. He decided to send a brigade of dragoons (15th and 25th Dragoons) to cross the Guarena downstream of the British line and outflank it, while Brennier's division launched a frontal assault from Castrillo, with Clausel's own division in support.
The cavalry attack was met by Victor Alten's brigade (14th Light Dragoons and 1st Hussars King's German Legion), which had been guarding the river downstream of the main lines. Alten let the French advance part-way up the hill and then attacked, hitting them before they could fully deploy. Both regiments were swept away and General Carrie, commander of the brigade, was captured. The French lost 150 men, including 94 prisoners, in this short fight.
The French infantry didn't do any better. Brennier crossed the river and advanced up the hill in three columns of regiments, with the battalions in rows behind each other. Wellington let them get part way up the hill, and then ordered W. Anson's brigade (3/27th and 1/40th Regiments of Foot) to attack, supported by Stubb's Portuguese brigade (11th and 23rd Regiments). Anson formed a longer line than the French and enveloped them on both sides. The French were outflanked and thus outgunned, and after a short fight turned and fled. Alten then attacked with his cavalry, taking another 246 prisoners. Clausel had to use part of his division to protect Brennier's retreating men.
This ended the fighting on 18 July. Marmont had lost around 700 men in the two combats, the Allies around 525. Marmont's great deceptive movement had ended in failure - the Allied rearguard had escaped, and Wellington was once again in a strong defensive position, protecting a good road to Salamanca and his communications.
The next few days saw the two armies continue to manoeuvre around each other, at one point marching in parallel for some way, before eventually Marmont made a crucial mistake, allowing Wellington to attack and defeat him at Salamanca (22 July 1812).