The Booker Prize first became open to American authors in 2014. That year, its eligibility rules were broadened from a regional system to include novels written in English by authors of any nationality, provided the books were published in the United Kingdom.
Before the change, the prize was generally restricted to writers from the United Kingdom, Ireland, the Commonwealth and, later, Zimbabwe. The reform made American novelists eligible for the first time and immediately changed the character of the competition. Four American writers appeared on the 2014 longlist: Karen Joy Fowler, Siri Hustvedt, Joshua Ferris and Richard Powers.
The 2014 prize was still won by an Australian author, Richard Flanagan, for The Narrow Road to the Deep North. The change therefore did not mean that American writers automatically dominated the award; it simply made them eligible to compete.
A useful distinction is that the Booker Prize did not become an American-specific award. It became international in author eligibility, while retaining requirements concerning English-language writing and publication in the UK or Ireland.