The Constitution has a method for new States to enter the union, it is silent on a States withdrawal. A federal court deciding in favor of the federal government, go figure. And the confederates didn't rebel and take it, they withdrew from the union and nationalized the assets with their territory. Much like the US did to indian lands.
.
Actually the Supreme Court ruled the Constitution declared secession invalid. Your disagreement with that judgement is irrelevant, it was still a rebellion.
And the Federal Court all the time rules against the government. We've seen it go against the Travel Ban of Trump and the recess appointments of Obama just for 2 simple examples. You can find plenty of them.
And again, you can't "nationalize assets" that are not yours. What you are saying they did is grounds for the US President to defend federal property.
What you are saying is you disagree with the Constitution. That's fine. That is your opinion if you disagree with the powers it gives. What that doesn't mean is that you get to say the Constitution is not the law of the land in the USA.