British rule in Ireland built upon the 12th century Anglo-Norman invasion of Ireland on behalf of the English king and eventually spanned several centuries that involved British control of parts, or the entirety, of the island of Ireland. Most of Ireland gained independence from the United Kingdom following the Anglo-Irish War in the early 20th century. Initially formed as a Dominion called the Irish Free State in 1922, the Republic of Ireland became a fully independent nation state following the passage of the Statute of Westminster in 1931. It effectively became a republic with the passage of a new constitution in 1937, and formally became a republic with the passage of the Republic of Ireland Act in 1949. Northern Ireland remains part of the United Kingdom as a constituent country.[1]