Secession is more aggressive than nullification, but nullification is just one step away from it. Secession creates a constitutional crisis which can result in a declaration of the Insurrection Act, and occupation by federal troops. That leads to something like Lexington and Concord eventually. Who fires the first shot in a civil war is mostly a matter of image. The underlying intractable political dispute was already present.
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With respect to your first sentence, I agree that federal troops could be deployed against militia groups loyal to the Seceding State. That will be where the rubber meats the road won’t it.
For our side to have a chance against a professional military we would have to have overwhelming numbers. Would our side show up to a fight? I don’t know. If we didn’t, then we must be able to take orders and lose most of our rights.
All this alarm over a States Convention is unwarranted. As a few pointed out it takes 3/4 approval to get anything done, and the battle to hammer out terms and language to assuage that super-majority takes time and much effort. But it should be started if only to get Congress closer to it's assigned track.
Those Founding Fathers were crafty sons of guns - it'll take more than knocking their statues down to dent their legacy. An honest judiciary would recognize the Commerce Clause has been stretched far beyond original intent just like the definition of citizenship. It's time for a revival meeting!