how did britain became a friendly country which was the arch anemy during the war?

2 answers

Friendly to whom and after which war?
If you mean Britain was the archenemy of the USA and France. Then lets check the two country out.(A quick breakdown)

USA fought two wars with Britain but due to WWI, Britain became allies with the USA for support and troops and the WWII as the Axis/Japan bomb Pearl Harbor.

France and Britain became allies over the Crimean War to prevent the Russians from invading Turkey. Then fought together in WWI and WWII plus in the Sue Canal.

Really, friendly to whom and what timeline? and after what event? Just like Damon said!