The combat of Castrejon (18 July 1812) was a rearguard action that came after Marmont outmanoeuvred Wellington on the River Douro, early in the campaign that ended at Salamanca.
In early June Wellington invaded Spain, advancing towards Salamanca. Marmont ordered his army to concentrate just to the north, leaving garrisons in the Salamanca Forts. These held out from 17-26 June (siege of the Salamanca Forts), triggering a series of attempts by Marmont to save them (including the combat of San Cristobal, 20-22 June 1812), but once the forts had fallen he decided to retreat to the Douro and wait for reinforcements to arrive.
The two armies were soon stretched out along the Douro. Wellington posted his left where the Tranancos joins the Douro, watching the ford of Pollos. His right was opposite Tordesillas, fifteen miles to the east.
Marmont held a longer stretch of the river. His left was near Simancas, seven or eight miles to the east of Wellington's right. At first his right faced the ford of Pollos, with Foy watching the river further west. On 7 July Bonnet arrived with 6,500 infantry, and was ordered to support Foy.
Marmont decided to try and outmanoeuvre Wellington. On 15 July Foy and Bonnet were ordered to take their divisions across the river at Toro, some way to the west of Pollos, while the rest of the army moved west along the north bank of the Douro.
Wellington fell for the ruse, and on the evening of 16 July began to move his army west to watch the road from Toro to Salamanca. The 1st and 7th Divisions were to move to Canizal and Fuente la Pena, west of the Guarena River. The right wing was also to move to Fuente la Pena (6th Division) and Canizal (5th Division). The left wing (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, moving to Castrejon on the Trabancos river, ten miles to the east of the main army.
On the night of 16-17 July Marmont reversed his direction of march. Foy and Bonnet were ordered back across the Douro, while the rest of the army returned east to Tordesillas. On the morning of 17 July Clausel and Maucune crossed the river there, followed by most of the rest of the army. Foy and Bonnet crossed at Pollos, and by the end of the day the French were concentrated at Nava del Ray, ten miles to the south-west of Tordesillas, and nearer to the Allied rear guard at Castrejon.
Wellington didn't realise that he'd been tricked until fairly late on 17 July. He immediately decided to head for the new front line himself, taking Bock's and Le Marchant's cavalry brigades with him. The 5th Division was ordered to advance to Torrecilla de la Orden, where they would be able to protect the retreat of the rearguard.
On the morning of 18 July Stapleton Cotton, the senior cavalry officer with the army, was in command of the rearguard. He sent out patrols to try and find the French, but they were soon forced back towards his main force. The French advanced towards Anson's cavalry brigade, which was lined up to the east of Castrejon. The French brought up two batteries of horse artillery and opened fire on Anson's men.
Cotton sent out Bull's and Ross's cavalry to try and deal with this threat, but they were badly outnumbered by the French cavalry. To make this worse, it soon became clear that Marmont had sent a large force of infantry towards Alaejos, to the north of the British line, with the intention of getting around the British left.
Wellington and Beresford arrived on the scene at about 7am, having got ahead of Bock and Le Marchant. They were caught up in part of the fighting on the left and briefly had to draw their swords, before British reinforcements forced the French cavalry to retreat.
Once this scare was over, Wellington ordered a full scale retreat towards the main army, picking up the 5th Division on the way. Anson's brigade was to cover the retreating infantry, while Bock and Le Marchant moved to the left to block Marmont's flanking column. The French were now advancing in two main columns, with their left attempting to catch the British rearguard, and the right attempting to reach the Guarena ahead of the retreating troops.
Most of the fighting came in the north, where the French right threatened the British 4th Division (Cole), at the northern end of the British line (the original left when the army was facing east, now really the right as the army moved west). The French even managed to get two gun batteries onto a knoll that overlooked the British route, and were able to open fire as the 4th Division marched past. General Cole used his divisional artillery and his light companies to form a screen on his right, and continued along his original route.
This was as a close as the French got to intercepting the British retreat, and all three divisions reached the Guerana safely. After a brief rest next to the river, the French began to appear on the hills east of the river, forcing Wellington to order his men to complete their movement into his new defensive position at Castrillo.
This ended the combat of Castrejon. Wellington was now in place to fight his preferred defensive battle, and for some time it looked as if Marmont was going to accommodate him, attacking the British left wing (combat of Castrillo, 18 July 1812), but this never developed into a full scale battle.