Didn't the Civil War settle this once and for all?

No. A war settles who had the stronger army. It does not settle what is legal, what is moral, or what a free people are entitled to decide for themselves. And what is being proposed today has nothing in common with 1861.

Force is not law

The idea that the Civil War "settled" the question confuses victory with legitimacy. By that logic, might makes right, and any question can be closed permanently by whoever wins the fight. We do not accept that anywhere else. The fact that the federal government had the power to hold the union together by force in 1865 tells us about its power. It tells us nothing about whether the people of a state have the right to govern themselves, which is a question of principle, not of arms.

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

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 is built on the consent of the governed, decided by a free, fair, and 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. Treating them as the same thing is either a mistake or a smear.

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

Since 1865, the principle of self-determination has become a cornerstone of international law, written into the United Nations Charter and into treaties the United States itself has 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 the question of self-government was frozen forever by a nineteenth-century war is a notion the rest of the world abandoned a long time ago.

The law does not say what the talking point claims

If leaving were truly settled and forbidden, there would be a clear constitutional prohibition to point to. There is not. The U.S. Constitution is silent on a state leaving the union, and under the Tenth Amendment, powers not delegated to Washington are reserved to the states and the people. The Civil War did not amend the Constitution to forbid independence. The talking point is asserting a settlement that the founding document never actually made.

The bottom line

The Civil War settled that the federal government would use force to keep the union together in 1865. It did not, and could not, settle whether Texans in the twenty-first century have the right to peacefully vote on their own future. They do.