Some people misunderstand Lincoln’s reluctance to cede federal territory in Fort Sumter. The fort was only secondarily placed there for defense of the harbor. The main reason it was there was to collect import duties and tariffs which were almost the sole means of funding the federal government in those days. Keeping the fort and keeping it garrisoned demonstrated an intent to continue to dominate the harbor and collect duties. In fact Lincoln even said South Carolina could do whatever they liked as long as duties continued to be collected. Obviously, no nation could allow a foreign power to collect import duties in its harbors.
Also, It was only federal territory in the sense that it was leased from the state of South Carolina. South Carolina leased the island to the federal government for a dollar a year in perpetuity for as long as they remained in the union. When they seceded they told the federal government they were canceling the lease and offered to pay for any improvements the federal government had made. When they fired upon it they did not view it as attacking the United States but rather reclaiming what was theirs. Jefferson Davis explains this in his book "The Rise and Fall of the Confederate Government".
