Are you serious? The US cannot invade its own territory, whether that be a US state, US territory, or US waters. Firing on a US government ship, fort, or troops by its citizens is an act of rebellion. The Civil War settled this. There is no right of secession for states. States are free to rebell from the US. However, the US is authorized to put down rebellions.Well, you see that the South's firing at Fort Sumpter was the declaration of war while I see that the invasion of the federal warship was a violation and a act of war.
Since we differ on that issue, there is no need nor should we argue against each other about that because that produces quarrels. I don't want to quarrel with anyone on differences in opinions about this great war subject.
From the Constitution:
Article I: The Legislative Branch: Section 8: Powers of Congress,
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions