By far the most popular sport in Kosovo is football. 1922 saw the founding of Kosovo's first clubs, including KF Vëllaznimi and FC Prishtina. During the Cold War era from 1945 until 1991, football in former Yugoslavia advanced so rapidly that in 1946, the Federation of Kosovo was formed as a subsidiary of the Federation of Yugoslavia. Prishtina were the nation's most successful club during that period, spending five years in the top-tier Yugoslav First League and reaching the semi-finals of the 1987-88 Yugoslav Cup. In 1991, an unsanctioned Kosovar league system known as the "Liga e Pavarur e Kosovës" ("Independent League of Kosovo") was set up, running parallel to the official Yugoslav leagues; in 1999, in the wake of the Kosovo War, this became Kosovo's official league system.