Rather, the war was made by the men who ordered cannon fire on US forces performing their duty at Ft. Sumter.
The 10th amendment does not reserve secession to the states. Rather it reserves powers to the states or to the people.
Secession could be legal, certainly by amendment, probably by legislation of the federal legislature, or by successful supreme court case, or by successful rebellion followed by treaty. The first three were not attempted by the slave owners of 1860, and they didn’t meet the high standard of the 4th.
I am not arguing that the Tenth makes secession legal, or that secession is called for, or that secession is imminent. I am arguing that to ignore the South’s arguments, or worse to say that they were evil, puts any argument for State authority over federal authority in peril.