Is it legal for Texas to leave the union?

Yes. On the law, the case is strong, and the objection is political dressed as legal. The Constitution does not forbid it, the only case cited against it concedes a lawful exit, Texas already meets the international definition of a nation, and the foundational text of Texas's own constitution affirms the people's right to decide. Here is the whole picture in one place.

The federal Constitution does not prohibit it

There is no clause in the United States Constitution forbidding a state from leaving the union. None. The Constitution is a list of powers granted to the federal government, and the power to prevent a state's departure is not on the list. Article I, Section 10 spells out exactly what states cannot do, and leaving is not among the prohibitions. Under the Tenth Amendment, every power not delegated to Washington is reserved to the states and the people. The power to stop Texas from leaving was never granted, so it was never Washington's to exercise.

The one case against us admits the door is open

The entire legal argument against Texas independence rests on a single 1869 opinion, Texas v. White. We treat that case in full elsewhere, but two points settle it here. First, its sweeping language about an "indestructible Union" was not the holding of the case, which was about who owned a batch of bonds; it was dictum, and the actual bond rule it delivered was overruled sixteen years later in Morgan v. United States (1885). Second, and decisively, the opinion itself concedes a lawful exit. Chief Justice Chase wrote that the union could be dissolved "through consent of the States." The case waved against us contains the answer inside it.

Texas already meets the international definition of a nation

Step out to international law and Texas more than qualifies. The Montevideo Convention of 1933, the foundational treaty defining statehood and now treated as customary international law, sets four criteria for a nation: a permanent population, a defined territory, a functioning government, and the capacity to enter into relations with other states. Texas meets all four, and not marginally. More than 31 million people. A defined territory of 268,597 square miles. A fully operating government. An economy that already trades with the world and runs its own foreign trade missions. Under the standard the world uses, Texas is already a state in every meaningful sense except the formal assertion of sovereignty.

International law contains no prohibition on declaring independence

The modern law reinforces the point. In 2010 the International Court of Justice held that "general international law contains no applicable prohibition of declarations of independence," and that territorial integrity is a rule between countries, not a cage for peoples. The United States argued for that result and recognized Kosovo the next day. Canada's Supreme Court held that a clear democratic vote creates a binding duty to negotiate. The direction of international law is consent of the governed, and Washington helped build that framework.

Texas's own constitution affirms the right, in plain words

Finally, the text that was written by Texans, ratified by Texans, and governs Texans. Article 1, Section 2 of the Texas Constitution reads: "All political power is inherent in the people. All free governments are founded on their authority and instituted for their benefit. The faith of the people of Texas stands pledged to the preservation of a republican form of government, and, subject to this limitation only, they have at all times the inalienable right to alter, reform or abolish their government in such manner as they may think expedient." At all times. Inalienable right. Alter, reform, or abolish. And Article 1, Section 1 still describes Texas as "a free and independent State." This has been the governing text of the Texas Bill of Rights for 150 years, and it contains no asterisk deferring to Washington.

The only real question is political, and it has a political answer

Put it together. The federal Constitution does not forbid it. The Tenth Amendment reserves the decision to Texas. The one case cited against it concedes a consent exit and rests on overruled dictum. International law contains no prohibition and points toward a duty to negotiate after a clear vote. Texas already satisfies the criteria for nationhood. And the Texas Constitution affirms the people's inalienable right to decide. What remains is not a legal barrier. It is the political work of passing the Texas Independence Referendum Act and winning the vote. Self-determination is a political question, and political questions have political answers.

The bottom line

Yes, it is legal for Texas to leave the union. The Constitution does not prohibit it, the precedent cited against it admits a lawful exit, Texas already meets the world's definition of a nation, and Texas's own constitution affirms the people's right to decide their future. The door is not locked. It never was. The work now is political: get the vote, and win it.