The Union was a name used by many to refer to the Northern states during the American Civil War. Besides the obvious fact that they were the remaining states left in the United States, the name seems also implied that the Southern states were disorganized, secessionist and separate. Since the term had been used prior to the War to refer to the entire United States (a 'union of states'), using it to apply to the non-secessionist side also carried a connotation of legitimacy as the continuation of the pre-existing political entity. Also, in the public dialogue of the United States, new states are "admitted to the Union."

Since the Civil War, the term has been mostly used as a synonym for the Northern side of the conflict, and has increasingly lost the more subtle connotations. So for instance, a modern person from the South (or elsewhere) who believes in the legend of the Noble Cause (also called the "Lost Cause") will often use 'Union', as in "The Union troops at ...."

In the United States, the issues of the Civil War have never entirely faded from ideological struggle, and occassionally resurface. Confederate apologists and States Rights supporters, for example, will often object to the term 'Civil War' and insist on 'War Between the States' or a similar term which implies (or allows inference of) legitimacy of the secession decisions in the 'tradition' of the Kentucky resolutions, or of the various New England secession movements in the early 19th century. The distinction is not merely formal, but is often connected with current controversies, as for instance support for a strong states' rights position. Although these positions have migrated over time between the two mainstream political parties and been championed by fringe parties throughout (see, for example, "Dixiecrat" and George Wallace), anti-Federalism is currently embraced by a segment of the Republican Party and the Libertarian Party.