Wellington was faced with a dilemma after his army liberated Madrid. Politically he could not fall back to Salamanca, but he faced the risk of being counter-attacked by a larger French forces from more than one direction.
The French had withdrawn their garrisons to Burgos and Valencia. According to Charles Esdaile they could field at least 100,000 men against the 60,000 of Wellington’s Anglo-Portuguese army, which might be increased to 70,000 by the addition of Spanish regulars. The guerrillas were good at harassing the enemy, but they could not resist a French counter-offensive.
In late August General Bertrand Clausel advanced on Valladolid, north of Madrid, intending to relieve the isolated French garrisons of Astorga, Toro and Zamora. Wellington, seeing a chance to defeat part of the French army before it concentrated against him, moved north with 21,000 men.
Wellington had taken what Esdaile describes as ‘a serious risk’ by moving with such a small force, but it was politically impossible for him to take more troops from Madrid. He hoped to receive support from General Fransisco Castaños’s Spanish 6th Army, which had just taken Astorgas.
In the event, the Spanish, who were short of supplies and artillery, moved slowly, and Clausel was able to retreat, taking the garrisons of Toro and Zamora with him.
Esdaile argues that Wellington should then have gone back to Madrid. He faced two French armies, and have could looked for an opportunity to win a major victory by concentrating against one of them. Instead, he decided to advance on Burgos.
The city was being pillaged by guerrillas, but a well supplied French garrison of veterans occupied a strong position in Burgos Castle. Esdaile compares General Jean-Louis Dubreton, its commander, to General Armand Phillipon, who had successfully defended Badajoz in 1811 and inflicted heavy casualties on the attackers before being forced to surrender the city in April 1812.
Burgos had nine heavy guns, 11 field guns and six howitzers. The garrison of 2,000 men had no permanent shelter. Frederick Myatt argues that the fortress could resist guerrillas or an army without siege guns but not an army well equipped with a siege train and engineers. 
However, Wellington had only three 18 pounder guns, five 24 pounder howitzers, five engineer officers, eight Royal Military Artificers, 10 assistant engineers and 81 tradesmen. His army was short of tools, although it found some French ones in the town.
Burgos was invested by the British 1st and 6th Divisions and two Portuguese brigades on 19 September. The 5th and 7th Divisions were positioned to the north-east to guard against a French attempt to lift the siege.
Wellington’s shortage of artillery meant that he had to concentrate on digging and mining, with his guns being used mainly to support assaults. An outer works, the Hornwork, was captured on the first evening of the siege, but at the cost of heavy casualties; 421 Allied compared with 198 French dead, wounded and captured according to Jac Weller. Seven French field guns were taken as well as 60 prisoners.
An attack on the castle’s outer wall on 22 September failed, and Wellington then concentrated on mining. The miners had to operate under fire from the castle, with little support from their own guns, and it often rained.
By 29 September the miners believed that they had reached the scarp wall and a mine was detonated that evening. The subsequent attack failed after troops became lost and failed to find the breach. In the morning it was revealed that it was not a good one, and that the French were working at shoring up their defences. The mine had been detonated too far forward, as the foundations that the miners had met were ancient ones rather than the those of the wall.
The failure damaged Allied morale; Myatt points out that the French had:
‘the reasonable hope that relief would arrive. The British…[were] feeling (perhaps rightly) that they were attempting a hopeless task with quite inadequate support’
The British worked on a second mine. They also set up a battery 60 yards from the outer wall, which was ready by 1 October. The French moved their guns to deal with this new threat and destroyed the battery the same day. The damaged guns were withdrawn and a new position prepared that night. The French artillery destroyed it before the guns could be moved into it.
It was intended to detonate the second mine on 3 October, but problems with the rocky ground meant that it was not ready until the next day. British guns made a breach 60 feet wide in the wall, which was extended to 100 feet when the mine detonated. The British attack succeeded in taking the breach with relatively light casualties.
Preparations now began for an assault on the second wall, but these were hampered by French sorties and poor weather. The attack was planned for 17 October, but Wellington delayed it for a day as he thought that the breach made in the second wall was inadequate. A third mine was detonated underneath the church of San Roman.
The French beat off the attack on 18 October. Wellington had, according to Jac Weller, 24,000 Anglo-Portuguese troops and 10,000 Spaniards around Burgos. He was now faced with 53,000 French soldiers commanded by General Joseph Souham, who had replaced Clausel. Another French army was advancing on Madrid from Valencia.
Wellington therefore called off the siege and withdrew on the night of 21 October. The French suffered 623 dead, wounded and captured during the siege, but inflicted 2,059 casualties on the besiegers.
Wellington’s rearguard fought an action against French cavalry at Venta del Pozo on 23 October. He initially hoped to make a stand along the River Carrión forty miles to the north-east of Valladolid and to join up with General Sir Rowland Hill’s corps from Madrid.
A series of engagements took place between 25 and 29 October, known collectively as the Battle of Tordesillas. The French captured the bridge over the Carrión at Palencia on 25 October and the bridge over the Duero at Tordesillas on 29 October. Wellington was therefore forced to retreat and ordered Hill to do the same
Hill had been preparing to fight a battle against the advancing French, commanded by Marshal Nicholas Soult. Instead, his rearguard fought an action against the French at Aranjuez on 30 October and he abandoned Madrid the next day. Wellington and Hill combined near Salamanca on 8 November and took up a strong defensive position. The French arrived six days later.
Soult moved to the west to threaten Wellington’s communications with Ciudad Rodrigo. Marshal Auguste Marmont had tried a similar manoeuvre in June and had been defeated after being caught on the march. Soult avoided this by staying further away from Wellington.
This left Wellington with the options of attacking a force that outnumbered him 95,000 to 70,000 or retreating. He chose to retire to Ciudad Rodrigo; it started to rain heavily just after the withdrawal began.
Esdaile says that ‘the French pursuit was none too vigorous.’ However, the Allies still lost 6,000 killed, wounded and missing. They included Sir Edward Paget, Wellington’s newly arrived second-in-command, who was captured on 17 November. Discipline and morale broke down as the troops retreated in bad weather, echoing the retreat to Coruña in 1809.
Wellington had lost much of the ground that he had won earlier in the year. However, the Allies still held the fortresses of Ciudad Rodrigo and Badajoz, known as the keys to Spain. An army invading Spain from Portugal had to hold these, so Wellington had a better starting point for his 1813 offensive than he had possessed in 1812.
 C. J. Esdaile, The Peninsular War: A New History (London: Allen Lane, 2002), p. 409.
 Ibid., p. 410.
 Ibid., p. 411.
 F. Myatt, British Sieges of the Peninsular War (Tunbridge Wells: Spellmount, 1987), p. 134.
 J. Weller, Wellington in the Peninsula 1808-1814, New ed. (London: Greenhill, 1992), p. 236.
 Myatt, British Sieges, pp. 136-37.
 Ibid., p. 142.
 Weller, Peninsula, pp. 237-38.
 Esdaile, Peninsular War, p. 418.