From enemies to close allies

The Irish Free State and Britain during the Irish civil war, which happened about 10 minutes after the Irish war of independence v the Brits!
 
The United States and Vietnam is fairly quick for modern times: from enemies in 1975 to exchanging diplomats in 1995 (I think) to Vietnam being a pretty important economic and military ally in the "War on Terror" with a US aircraft carrier making a friendly visit to Ho Chi Minh City a year or two ago.
 
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