Isonzo or Soča, river, south-eastern Europe, rising in the Julian Alps in north-western Slovenia and flowing south into north-eastern Italy. About 140 km (87 mi) long, the river turns sharply south-east near Gorizia, on the Italian border, and empties into the Gulf of Trieste, an arm of the Adriatic Sea. During World War I the Isonzo Valley was the site of many battles between the Italian and Austro-German forces and was included within Austrian territory ceded to Italy after the war. By the terms of the Treaty of Paris in 1947, Italy ceded to Yugoslavia that part of the river north of Gorizia. Known as Soča in the Slovenian language, the river became part of independent Slovenia in 1991.