The Slavery System was a failure, semi ,- Socialistic and inherently anti- free market. Leading Southerners actually referred to the Slave Economy as Socialist. The US Government was small before before the war.
If it was a failure, it would have collapsed on its own. It wouldn't need a million man army to stop it.
It was in fact, quite successful economically, but it wasn't going to be tolerated after it became a threat to the Washington DC establishment.