The Siege of Ansi was a battle between Goguryeo and Tang forces in Ansi, a fortress in the Liaodong Peninsula, and the culmination of the First campaign in the Goguryeo–Tang War. The confrontation lasted for about 3 months from 20 June 645 to 18 September 645. The initial phase of combat resulted in the defeat of a Gorguryeo relief force of 150,000 and resulted in the Tang troops laying siege to the fortress. After a siege lasting around 2 months, the Tang forces constructed a rampart. However, the rampart was on the brink of completion, when a section of it collapsed and was taken over by the defenders. This, along with arriving Goguryeo reinforcements and the lack of supplies, forced Tang troops into a retreat. Over 20,000 Goguryeo troops were killed during the siege.