[lentulusgracchus]

Why did Lincoln want to retain Sumter? Partly because he said he would do so on several occasions, including his inaugural address. But in large part because that by giving into southern demands not to reinforce Sumter or, worse, ceding Sumter to the Davis regime, he would be giving them de facto recognition as a sovereign nation. And that is something he would never do.

And because he wanted a casus belli that would exonerate him, in his own and the public's eyes, of what he was about to do: solve the slavery issue his way, by bloodshed.

[Non-Sequitur]

Lincoln was out to preserve the Union at almost any cost. He was even willing to support the 13th Amendment passed by Congress in 1861 if that would help preserve the Union. If anyone was looking for a casus belli at Sumter it was Jeff Davis.