Final answer:
Ireland gained independence from the UK in 1921.
Step-by-step explanation:
The country that gained independence from the UK in 1921 is Ireland. After years of political and social unrest, the Anglo-Irish Treaty was signed in December 1921, which established the Irish Free State as a dominion within the British Empire.
However, it's important to note that full independence was not achieved until the establishment of the Republic of Ireland in 1949.