How to treat a country name that has a plural meaning

There are many cases where an entity that's made up of plural words is singular. Obviously, the United States of America is a single entity, so "has" is right. This isn't an exception; this is the rule.

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Strictly speaking, you should handle it on a case-by-case basis. In everyday usage (i.e., probably anything short of official diplomatic correspondence), stick with the singular.

(Background: Before the American Civil War of 1861-65, it was customary to say "The United States are...". Before the war, the US was a much looser federation of sovereign states than it is today, culturally as well as politically: a resident of Virginia would have considered herself a citizen of Virginia first and of the United States second. The war greatly strengthened the national bonds between the states, even to the point of changing the language: after the war and ever since, people have said "The United States is...". So there's precedent for treating a single nation-state as plural, but in the modern world it's not something that's likely to come up.)