You’re missing the point (I suspect intentionally). The south knew that it was a matter in contention. They knew that they could choose negotiation or confrontation and war. They chose war. Doesn’t that make them advocates of “might makes right”?
“The south knew that it was a matter in contention.”
You are right there. But being a matter in contention is far different than being something something “expressly unconstitutional.”
And too, Southerners knew about the speech given on the floor of the U.S. House of Representatives in 1847 in which Congressman Abraham Lincoln said: “Any people, anywhere, being inclined and having the power, have the right to rise up and shake off the existing government, and form a new one that suits them better. This is a most valuable, a most sacred right, a right which we hope and believe is to liberate the world.”