The guy is naïve. If there is a war of succession and the federal government cannot re-establish control, tactical nuclear weapons will be used on a limited basis.
Case in point. AOC becomes president of the US in 2030 (when she is 40), hard left socialism is implemented, the economy collapses (as in Venezuela) and the deep South succeeds (which it should in this case). Generalized guerilla warfare occurs which upends federal control of several states without any hope of reestablishing control by the Federal government. You can bet your bottom dollar that AOC uses tacticals in a heart beat. Just think of Sherman's march to the sea which was nothing more than the federal governments own version of a tactical nuclear weapon in its day and age.
There is no limit to violence that a government will resort too to maintain control. McClelland was reticent, Grant was not.