It was on their turf to start with. It was the US army in the US.
Of course the south would argue that it ceased to be part of the US, when the sourthern states passed resolutions stating that they are leaving the union. But the right to do so was not clear cut, and even if they have the right to do so they were pretty quick to attack, it wasn't as if they exhausted all other possibilities over an lengthy period of time.
CAS formed Feb. 1861. CSA starts taking control of federal instalations in the south right away.
March 4 1861
Lincoln "The power confided to me," he pledged, "will be used to hold, occupy, and possess the property and pl aces belonging to the Government." He added, however, that beyond what was necessary to achieve this objective, "there would be no invasion, no using of force against or among the people anywhere." He said nothing about reclaiming property now under Confederate control, and he explicitly offered reassurances to the South that slavery was safe in its present limits and that he would enforce the fugitive slave clause of the Constitution. But he held that the Constitution and the Union were perpetual, that secession was, therefore, legally void, and resistance to the federal government constituted rebellion
The Attack on Fort Sumter (April 12-13, 1861)
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