“The potential cost in lives and treasure alone would mitigate against such an outcome.”
The Union side might think the secessionist side believes that, and that the mere threat or bluff of force would make them fall back in line.
They might be right.
Or they might not be right and things could get out of hand with neither side being willing to back down.
When the War Between the States began, neither side thought it would turn out to be as long and hard as it did turn out to be.
I said there'd be all kinds of smoke and hot air out of DC if one or more states decide to secede. But that's all it'll be, and people in the secessionist states will know it.
No one is going to launch a hot war of blood and destruction over any state seceding.