“The Confederates had already taken over many other forts, yet none of these were regarded as a Casus belli.”
I don’t claim to be a ‘Civil War Expert’, my study has been limited to the Western Front, in Indian Territory, Texas & Arkansas. But you are correct. Ft. Smith was taken without a battle April 23rd, 1861 by Arkansas troops and Stand Watie and his Cherokee Braves. Fort Washita was abandoned by Union Forces April 16. As early as Feb 8th, was the seizure of Little Rock Arsenal by State Troops.
I’ve found the Civil War was fought for a lot of ‘local’ reasons. Just as I suppose any war of such a great geographical scale would be.
http://jesusweptanamericanstory.blogspot.com/
By giving up those Forts, it established the precedent of doing so, and an acknowledgement that the USA recognized the Confederate States claims on them. From what I have been reading, Washington was sending mixed signals about giving up on Fort Sumter and Fort Pickens as well.