The diversity of ethnic and religious groups has not changed dramatically since 1920, and although relationships between these groups have improved, not all problems have disappeared. Throughout the 20th century there was tension between Sunni and Shi’i and continual fighting between Kurds and Arabs. Iraqi Kurds were given autonomous control of Kurdistan, a federally recognized region in northern Iraq, in 1991. However tensions are running high once more, partly due to the recent discovery of massive oil reserves in Iraqi Kurdistan. The 2003 invasion of Iraq exacerbated Sunni-Shi’i conflict, and many areas of the country, particularly those with mixed populations, have become increasingly unstable.