...Historians debate what Lincoln's motives [were].

Sometimes a cigar is just a cigar.

One of the more recent and best, if I may say so, discussions of the whole run-up to the attack on Fort Sumter can be found in Chapter 12 of Doris Kearns Goodwin's Team of Rivals.

She makes a convincing case that Lincoln, in attempting to re-supply Fort Sumter (and Fort Pickens) was simply fulfilling the pledge made in his Inaugural Address to "hold, occupy and possess" property belonging to the Federal Government, but "beyond what may be necessary" to accomplish that there would "be no invasion, no using of force."

The message Lincoln sent to the Governor of South Carolinas speaks for itself: "I am directed by the President of the United States to notify you to expect an attempt will be made, to supply Fort- Sumpter with provisions only; and that, if such attempt be not resisted, no effort to throw in men, arms, or ammunition, will be made without further notice."