Istria (ĭsˈtrēə) [key], Croatian Istra, mountainous peninsula c.1,500 sq mi (3,900 sq km), in Slovenia and Croatia, projecting into the N Adriatic between the gulfs of Trieste and Fiume. A section of the northwestern portion, including the city of Trieste, belongs to Italy. The area is thickly forested and is predominantly agricultural. Pula is the chief city and a shipbuilding center. The population is about two thirds Croatian. Istria was inhabited by Illyrian tribes when it passed (2d cent. B.C.) to Rome. It remained under nominal Byzantine rule until the 8th cent. A.D. By that time, Slavs had settled in the rural areas and Italians in the cities. By the 15th cent. Austria and Venice had absorbed, respectively, the northeastern and southwestern parts of the region. The Treaty of Campo Formio (1797) and the Congress of Vienna (1815) added the Venetian part to Austria. In 1919 all Istria passed to Italy, but the Italian peace treaty of 1947 gave most of it to Yugoslavia. The northwestern section passed to Italy in 1954; under the 1975 Osimo Treaty, Italy gave up claims to coastal lands south of Trieste.