You most likely already have an opinion on this, but let’s make it more interesting: let’s assume that secession was, in fact, authorized by the Constitution.
So I ask again, did the seceding states commit treason? Before you answer, however, consider this fact:
At least six of the seceding states (FL, AL, GA, LA, AR, NC) seized federal property by force of arms prior to seceding from the Union, that is, before those states passed ordinances of secession.
South Carolina December 20, 1860
Mississippi January 9, 1861
Florida January 10, 1861
(January 6, 1861: Florida seizes Apalachicola arsenal)
(January 7, 1861: Florida seizes Fort Marion)
Alabama January 11, 1861
(January 4, 1861: Alabama seizes U.S. arsenal at Mount Vernon)
(January 5, 1861: Alabama seizes Forts Morgan and Gaines)
Georgia January 19, 1861
(January 3, 1861: Georgia seizes Fort Pulaski)
Louisiana January 26, 1861
(January 10, 1861: Louisiana seizes U.S. arsenal at Baton Rouge, and Forts Jackson and St. Philip)
(January 11, 1861: Louisiana seizes U.S. Marine Hospital)
(January 14, 1861: Louisiana seizes Fort Pike)
Texas February 1, 1861
Virginia April 17, 1861
Arkansas May 6, 1861
(February 8, 1861: Arkansas seizes U.S. Arsenal at Little Rock)
(February 12, 1861: Arkansas seizes U.S. ordnance stores at Napoleon)
Tennessee May 7, 1861
North Carolina May 20, 1861
(April 14, 1861: NC secessionists seize Fort Macon)
So the question I put before you now is: did those six states commit treason?
Before you answer that question, consider this: in 1859, John Brown was tried and hanged by the state of Virginia. Why? Brown had attempted to seize federal property—namely, the federal arsenal at Harpers Ferry. If you believe that southern states had the right to seize federal property prior to secession, but also believe that John Brown was a traitor who deserved his fate, how do you reconcile those opinions?
More info on this interesting topic:
Confederate Confiscations of Federal Property
Did Virginia Commit Treason?