[TexConfederate1861]

The legal basis is as follows:

The legal landlord filed and served an eviction.
The tenants instead of leaving, chose to call for some tough guys for help.

The landlord called in the constables who EVICTED the tenants forthwith. Simple enough explanation.

Secession was legally voted for and passed by the people of South Carolina, they LEFT the Union, therefore any installations on THEIR property became THEIR property by default. (and don't tell me they didn't LEAVE, because that dog won't hunt either.) They had to be RE-ADMITTED, so obviously they LEFT.

[Non-Sequitur]

The legal landlord filed and served an eviction.

Fell apart right there. South Carolina was not the legal landlord. Fort Sumter was built on land deeded to the federal government free and clear by an act of the South Carolina legislature. Constitutionally it could not have been otherwise. Therefore, South Carolina had no legal claim to the property at all.

...therefore any installations on THEIR property became THEIR property by default.

Based on what rule of law?