Actually he did.
On April 12, Maj. Anderson told Charleston authorities the following: he “would evacuate on the 15th….”.
You said, “There was no invasion…”.
Then, what was this:
On April 6, Lincoln ordered a naval expedition to ostensibly resupply Fort Sumter.
This became a federal act of war against the South! The expedition consisted of the armed side wheel steamer HARRIET LANE, the second-class screw sloops PAWNEE and POCAHONTAS, the large paddle- wheel sloop POWHATAN, the transport BALTIC with 200 soldiers and provisions, and three tugs, the FREEBORN, UNCLE BEN, and YANKEE. The ships were to sail individually to preserve the secrecy of the expedition. The tugs had most of their machinery below the waterline and any above was protected by hay or cotton bales. Upon arriving at Charleston Harbor, they were supposed to pass about 1300 yards from the land batteries and being on a cross course and moving in the darkness at 14 knots they would hardly be hit. If hit, however, there were sufficient launches to save the personnel that Lincoln told Governor Pickens would not be present. Launches were to be used if the seas are calm.
“Then, what was this:”
It was what it was. A small relief expedition going to Fort Sumter. Three sloops, hardly a major battle force. Besides, with 200 soldiers and provisions that’s not much of an invasion.