Did the Civil War make secession illegal?

No. A war can decide who has the stronger army. It cannot make a law, change the Constitution, or settle what a free people are entitled to decide for themselves. And what is being proposed today has nothing in common with 1861.

A battlefield is not a legislature

The claim that the Civil War "made secession illegal" confuses winning with lawmaking. Laws are made by writing them down and enacting them, not by winning a battle. Ask the simple question that takes the argument apart: which clause of the Constitution did the war amend? There is no answer, because the war amended nothing. The Constitution after 1865 says exactly what it said before, and it contains no prohibition on a state leaving. The federal government had the power to hold the union together by force. That tells you about its power. It tells you nothing about the right of a people to govern themselves.

Conquest cannot settle a question of right

If military victory could permanently settle a question of right, then every question of right is decided by whoever wins the fight, and might makes right. We reject that everywhere else. When Germany invaded Poland, the world did not say "well, that settles that." Conquest has never been a substitute for consent. The idea that a nineteenth-century war permanently answered a question of self-government is a principle no free people would accept in any other context.

1861 and today are not the same thing, and the difference is everything

This matters too much to blur. The secession of 1861 was bound up with slavery and was carried out by conventions, not by a vote of the whole people. Texas independence today rejects all of that. It rests on the consent of the governed, decided by a free, fair, open referendum of every eligible Texan, of every race and background. One was a war to preserve human bondage. The other is a peaceful vote to secure human freedom. Anyone who collapses the two is either confused or trying to smear a lawful movement by tying it to the worst chapter of the past.

Even the courts the war supposedly settled kept arguing

If the war had truly ended the legal question, no court would have needed to address it again. Yet the law kept moving. Texas v. White itself, four years after the war, conceded the union could end "through consent of the States." Decades later the Supreme Court held in Jacobson v. Massachusetts (1905) that the Preamble, the very text used to claim a permanent union, grants no power at all. The legal ground has shifted away from the "settled by war" claim, not toward it.

The world moved on, even if the talking point did not

Since 1865, self-determination has become a cornerstone of international law, written into the United Nations Charter and into treaties the United States itself signed. Free peoples have voted on independence in Scotland, in Quebec, across the former Soviet Union, and in Britain's exit from the European Union. The notion that a question of self-government was frozen forever by a nineteenth-century war is a notion the rest of the world abandoned long ago.

The bottom line

The Civil War settled that the federal government would use force in 1865. It did not, and could not, settle whether Texans in the twenty-first century may peacefully vote on their own future. They can, and no battle a century and a half ago changes that.