The term "Union" has been used interchangeably as shorthand for the United States since its inception, and was used throughout the nation much like "The Republic" or "America." Examples: The Articles of Confederation and Perpetual Union, "We the people, to form a more perfect union," "The Union must and shall be preserved," "The Union is dissolved," etc... While being both singular and plural, the term referred to the United States and everyone at the time understood it as such. Its use, as JackO notes in his post above, was simply a shorthand way of referring to the United States. During both world wars the United States Army was referred to as the "American Army" the same way and it wasn't regarded as improper or inaccurate, just not official.