The easiest way would be to use GeoDNS. You just have your DNS server send out a different IP according to the IP of the request.

See http://www.caraytech.com/geodns/ and https://en.wikipedia.org/wiki/Geodns

I've found this nice answer on StackOverlow that show the differences between Anycast, GeoDNS and using an http redirect: http://stackoverflow.com/a/25678199