Man skriver det i entalsformen:
The United States is a big country with many states
Undtagelsen er, hvis du bruger ejefald, så skrives United States i flertalsform. Det vil sige United States' og ikke United State's. For eksempel:
The United States' role in the Second World War had a big impact on the rest of the world.
PS: For de historieinteresserede kan jeg oplyse, at for ca. et århundrede siden brugte man flertalsformen af United States.