>>1
Sounds like "horizontal allegiance" is supposed to mean allegiance to a group of peers (like (members of) one's city or state) as opposed to (central) authority figures like the POTUS.
If so, it's a fancy-pants way of saying ppl started listening to their own state folks instead of the high-horse brass in Washington, or something. People's loyalties were getting decentralised, and they wanted to do their own thing.
Which would be one driving force behind the secession of the South. Which is, obviously, what triggered the Civil War.