(Flemish, leper), a city in northwestern Belgium, in the province of West Flanders. Population, 18,500 (1967). It is the site of textile and food industries. Machines used in the production of textiles are also manufactured. The city was first mentioned in chronicles in 1109. Between the 12th and 14th centuries, Ypres competed with Bruges and Ghent as the center of the production of cloth by guilds. Later the city went into a decline. Architectural landmarks in Ypres include the Cathedral of St. Martin (13th-15th centuries) and Cloth Hall (1200–1304), a masterpiece of secular Gothic architecture. Other monuments of medieval Lowlands architecture have also been preserved.
During World War I (1914–18) in the vicinity of Ypres, the German Fourth Army launched a counterattack against the Ypres salient (April 1915) and forestalled an offensive being prepared by the Anglo-French forces. The Germans occupied most of the salient. On April 22, the first day of fighting, the Germans used a weapon of chemical warfare (chlorine gas) and inflicted heavy casualties on the enemy. This was the first use of chemical warfare in the history of war. In 1917, from July 7 to November 6, the English Fifth and Second armies and the French First Army repeatedly launched offensives. Achieving minor successes, they suffered enormous losses. On July 12, 1917, the Germans used mustard gas, which is also known as yperite, for the first time.