Ireland was part of the United Kingdom from 1801 to 1922. For almost all of this period, the island was governed by the UK Parliament in London through its Dublin Castle administration in Ireland. Ireland underwent considerable difficulties in the 19th century, especially the Great Famine of the 1840s which started a population decline that continued for almost a century. The late 19th and early 20th centuries saw a vigorous campaign for Irish Home Rule. While legislation enabling Irish Home Rule was eventually passed, militant and armed opposition from Irish unionists, particularly in Ulster, opposed it. Proclamation was shelved for the duration following the outbreak of World War I. By 1918, however, moderate Irish nationalism had been eclipsed by militant republican separatism. In 1919, war broke out between republican separatists and British Government forces. Subsequent negotiations between Sinn Féin, the major Irish party, and the UK government led to the signing of the Anglo-Irish Treaty, which resulted in five-sixths of the island seceding from the United Kingdom, becoming the Irish Free State (now the Republic of Ireland), with only the six northeastern counties remaining within the United Kingdom.