Secession dis not cause the war. Secession WAS NOT and is STILL NOT illegal. To be sure I never read a speech or proclamation from either Buchanan or Lincoln stating that the war was brought on because secession was illegal. In fact S. P Chase made this unverified statement---

No law had ever been passed that explicitly outlawed secession, the argument simply having been sidestepped by events. From the legal point of view it would have been difficult to accuse Mr. Davis of having committed any crime. Judge Chase felt there was no strong legal case against him for having been the president of the Confederacy, and added, with a surprising wisdom: "Lincoln wanted Jefferson Davis to escape, and he was right. His capture was a mistake. His trial will be a greater one. We cannot convict him of treason. Secession is settled. Let it stay settled."

The Chase quote is from "The Long Surrender", 1985, by Burke Davis
His sources are..
Southern Historical Papers, Vol. 37, pp 244-52
"Why Jefferson Davis Was Never Tried" by George S. Boutwell, is in ibid., Vol. 38. pp. 347-49.
"The U.S. vs. Jefferson Davis," by Ray F. Nichols, American Historical Review, Vol. 3, No. 2, January 1926, pp 266 ff.
"The Trails and Trail of Jefferson Davis," a paper read before the Virginia Bar Association 1900, and published by this organization.
******************************************

Please note I said unverified. I say this because I have been unable to chase down these sources cited and verify by first or second sources.

GP