Aire-sur-la-Lys is a commune in the Pas-de-Calais department in northern France.GeographyThe commune is located 10 miles southeast of Saint-Omer, at the junction of the N43 with several departmental roads, by the banks of the Leie and the Laquette rivers.HistoryAire-sur-la-Lys is mentioned for the first time in 857. It developed around a fort or castrum built by Baldwin II, Count of Flanders in response to the Norman invasions. More growth followed with the establishment of the Collegiate church of Saint-Pierre by Baldwin V, Count of Flanders. The town was laid siege ten times between 1127 and 1710. It was separated from the County of Flanders and attached to the County of Artois in 1196. Subsequently ruled by the Burgundians then by the Spanish.  The town was besieged in 1676 by Vauban and retaken for France, although it remained a Spanish possession until 14 April 1713, when, by the Treaty of Utrecht, it finally became a part of France. Vauban’s stronghold, which was a strategic position, was dismantled in 1893.