Singular. Lots of threads on this in the archive (Suche in allen Foren). (-:

Also, 'the US' or 'the United States' is more common in English than 'the USA,' though all are okay.

Danke! Dachte mir schon, dass es diese Frage schon öfter gegeben habe muss... (Habe nur unter Stichwort USA gesucht, aber nichts gefunden.)

Depends on context. If you mean the country, it's singular. If you mean their soccer team, it's plural (like England, Germany etc.)

The United States have won the World Cup... !

As in
England have won the World Cup.

(or The Ashes, or ten Olympic golds, or whatever)

But: The United States is a major banana-importing country.

Escoville, stop muddying the waters! ;-))

I think that usage is primarily BE. I'm not into sports at all, but in AE we would say,
England has won the World Cup.
and similarly,
The United States has won the World Cup.

BE differs at times from AE with how groups of people are treated with respect to singular and plural verb forms.