Tip: See my list of the 
MostThe United States has always been causing (grammatical) trouble… or have been? “The United States” was treated as a plural noun in most of the 19th century, but the usage shifted during the 20th century towards treating it as a singular noun.
In other words, we understand “The United States” as “the country consisting of united states” in modern English and use singular verbs after it, as in
correct The United States has a very aggressive foreign policy.
obsolete The United States have a very aggressive foreign policy.
We can see the singular United States also in the following witticism by Winston Churchill:
The United States invariably does the right thing, after having exhausted
every other alternative.
every other alternative.
