In English, we say “The United States is”, not “The United States is”, so it might seem that the correct form in English should be “The United States are”, but this is not the case.
The term “United States” was indeed formerly understood as a plural, but during the twentieth century the usage began to change, and now we understand “The United States” as a shortened form of “the country consisting of united states”. Since “the country” is singular, we also use the singular verb with “The United States”:
- The United States has a very aggressive foreign policy. (correct)
- The United States have a very aggressive foreign policy. (obsolete)
Evidence of this usage can be seen, for example, in the following bon mot by Winston Churchill:
The United States invariably does the right thing, after having exhausted
every other alternative.