At the rime of its occupation, the island was nothing more than a reinforced stone building. It was never commissioned.
The notion that it was “union property” was a canard used by newspapers to incite people with limited knowledge of law.
Union military had already fired on the South and arranged a blockade of ports.
The officials knew the Union was invading and took steps to stop it.
Fyi
Fort Sumter was covered by a separate cession of land to the United States by the state of South Carolina, in a resolution passed by the South Carolina legislature in December 1836. The legislation did not specify that the Fort was to be used to protect the state. Fort Sumter belonged to the Unites States.
The Union blockades of Southern ports did not begin until *after* the battle of Fort Sumter. The Battle was in April, and the Union Blockade Strategy Board wasn’t even formed until June. The first shots of the Civil War were by the South—not the North.