United States is a singular noun and takes singular verb forms; for example, we say “The United States is in the Western Hemisphere,” not “The United States are in the Western Hemisphere.” This has been the case for over a century. Think of United States as the name of a country like any other. England, China, and Bahrain, for example, are all treated as singular nouns. United States is the same, even though it takes the form of a plural noun.
For example, these publications treat United States as singular:
According to estimates, the United States is set to hit its current debt ceiling between April 15 and the end of May. [Washington Post]
The United States has issued a rare public apology for the “repugnant” actions of a so-called American “Kill Team”. [Telegraph]
The United States is now actively pushing the United Nations Security Council to approve a wide range of military options that could be used against the Libyan regime. [National Post]
Possessive United States
Although United States is usually treated as a singular noun, it’s treated as plural when made possessive. United States’, not United States’s, is the preferred form:
Another delicate issue here is the United States’ role in the 1973 military coup. [Miami Herald]
Once we have driven Gaddafi from power it would then be the United States’ job to sort out who the rebels are and whom to put in power. [Global Post]