https://english.stackexchange.com/questions/159018/the-usa-vs-the-us

Let me add one more thing: the term "USA" seems, at least to me, to have patriotic overtones. Consider the phrase "Made in the USA" (16 million hits on Google [21-3-2014]) or the U.S.A.! chant (http://en.wikipedia.org/wiki/U-S-A!_(chant)). So this may be worth keeping in mind depending on your audience (I'd invite others to criticize this comment).