The Tag Synonyms to merge question currently contains only the suggestion to merge us and usa into united-states. While this is probably appropriate for the vast majority of questions asked with either tag, it is not technically correct, although no question so far deals with the country officially called the "United Mexican States".
How should this, and in general ambiguities with one, erm, dominant resolution, be handled? (wikipedia, for one, redirects United States to United States of America with a note and link to a disambiguation page on top.