Given that the United States Constitution nowhere prohibits secession, that phrase would indeed describe anyone who forcefully opposed the secession of the Southern States.
By the way, shall we discuss the secession of the States that ratified the Constitution, from the so-called "perpetual union" formed under the Articles of Confederation? When ever the subject pops up, you seem to make yourself scarce...
Ho hum.
It doesn't prohibit murder either. Secession is outside the law and all the bleating in the world won't change that.
Walt