In his Inaugural Address, Lincoln stated that it was his duty “to collect the duties and imposts,” but beyond that “there will be no invasion of any state.”
What that really meant was that he was saying that if the seceded states failed to collect and pay the newly-doubled tariff rates, as the South Carolinians did with respect to the 1828 Tariff of Abominations, there would be an invasion.
He was true to his word.
4/8/1861 Armed Coast Guard revenue cutter, Harriet Lane, put out to sea from New York with sealed orders.
4/9/1861 The steam transports Illinois and Baltic got to sea. The Illinois carried 300 troops and the Baltic had 200. The Powhatan had sailed on the 7th with 358 troops. The Pawnee left from Norfolk with 10 guns and 200 troops.
He instructed Fox to fight his way in if opposed.
Fighting is vicious work.
Less than 1100 troops total, when the CSA and SC forces in the area were over 6,000, not to mention tens of thousands more that could quickly have been brought to bear.
Yup, sounds much more like a definitive plan to invade and conquer SC than a plan to reinforce and hold a fort owned by the USA.