The siege of Astorga (2 July-18 August 1812) saw the Spanish attack the isolated French garrison of Astorga, in an attempt to support Wellington's advance to Salamanca.
Wellington attempted to put in place a number of secondary campaigns to prevent the French from uniting their much larger armies against him. One of them was for the Army of Galicia, then commanded by General Abadia, was to besiege Astorga, a move that would threaten the French troops in the Asturias, and stop them from helping Marmont, his main opponent around Salamanca.
Wellington also hoped that the siege would distract Bonnet, whose division had moved into the Asturias in mid-May, and prevent it from returning to Marmont's main army, but on this occasion Marmont ignored Napoleon's orders and called Bonnet back as soon as he realised how large Wellington's army was.
General Abadia wasn't keen on the plan, but his superior, General Castanos, was more enthusiastic. He moved to Santiago to make sure the attack got underway, but he decided not to take command of the field army, and instead gave that to General Santocildes.
Santocildes had around 15,000 men at his disposal, and the only French troops left in his area were the 1,500 men in the garrison of Astorga (two battalions from the 23rd Light and one from the 1st Line). Wellington wanted him to besiege Astorga with around 3,000-5,000 men, but send a strong force of 10,000-12,000 towards Marmont to force him to divert some troops in that direction. Instead Santoclides blockaded Astorga with most of his men, and only pushed 3,800 forward to the River Esla, where they were still too far from Marmont to worry him.
The siege itself wasn't conducted with any urgency. During June Abadia and then Castanos claimed that they didn’t have any siege guns, so the place was only blockaded. Sir Howard Douglas then reminded them that they had a number of heavy guns at Corunna, and they were finally able to bring up a proper siege train, which arrived on 2 July.
Once the guns were in place, Santocildes announced that he expected Astorga to fall within a few days. The French only had enough supplies to last until 1 August. After that they were forced onto half rations, and later onto quarter rations. News of the French defeat at Salamanca reached the garrison, but didn't convince them to surrender.
The siege came to an end just as the French were on the verge of lifting it. General Clausel, the new commander of the French Army of Portugal in the aftermath of Salamanca, was able to restore order in his army surprisingly quickly. He decided to launch a raid into Wellington's rear area, in the hope of lifting some of the pressure on King Joseph, and also to save the besieged garrisons of Toro, Zamora and Astorga. Foy's division was detached to carry out this second task. It reached Toro on 17 July, lifted the siege and destroyed the fort. Foy then moved towards Astorga, aware that its garrison was the most vulnerable. However on 18 August, having already lost 300 men, and with no idea that Foy was close by, the garrison surrendered. Castanos then withdrew back into Galicia, unwilling to risk a battle with Foy's experienced troops.
Foy reached La Baneza, sixteen miles from Astroga, on 20 August, where he discovered that the garrison had already surrendered. His cavalry reached Astorga on 21 August and found the fort intact but empty apart from seventy sick soldiers, who had been left behind. Foy then turned towards Zamora, forcing the lifting of that siege as well.