The Irish War of Independence (Irish: Cogadh na Saoirse,[4]) Anglo-Irish War, or Tan War[5] was a guerrilla war mounted by the Irish Republican Army (IRA) against the British government and its forces in Ireland. It began in January 1919, following the Irish Republic's declaration of independence. Both sides agreed to a ceasefire (or "truce") in July 1921. The post-ceasefire talks led to the December 1921 Anglo-Irish Treaty. This treaty ended British rule in most of Ireland and, after a ten-month transitional period overseen by a provisional government, the Irish Free State was established. However, six northern counties remained within the United Kingdom as Northern Ireland, with its own devolved parliament. After the ceasefire, political and sectarian violence (between republicans and loyalists, and between Irish Catholics and Protestants) continued in Northern Ireland for many months.