>>My understanding is that Texas entered into this mutated Union as a Sovereign Republic (a free country). If they want to secede from a freely entered union, they should be completely free to leave it. <<
That is urban myth. Upon statehood any territory becomes the same as any other state. The War of Northern Aggression cemented that relationship.
That kind of assumes that Texas is looking for anybody's permission to secede.
Texas was not a territory when it joined the Union.
Resolution Annexing Texas to the United States
(1 March 1845)
The annexation of Texas was a key issue in James K. Polk’s U. S. presidential election campaign of 1844. As a result, Polk’s victory that November was interpreted in the United States as a mandate to annex the ten-year old republic. Early the following year, a joint resolution for annexation passed both houses of the U. S. Congress—even before Polk’s inauguration. It was signed by outgoing President Tyler on March 1, 1845, subject to acceptance by the Republic of Texas.
The following summer, the Convention of 1845 met in Texas to consider the annexation issue. By a vote of fifty-five to one, the convention accepted the annexation offer by passing an Ordinance with virtually the same wording as the U. S. resolution.
The resolution, as shown below, contains several interesting provisions unique among the states. Under the agreement, Texas was to retain all of its “vacant and unappropriated lands,” as well as its public debts. In addition, Texas, which claimed a land area about 50 percent larger than that of the present state, was given the option to form out of its territory up to four additional states. The issues of land boundaries and public debt, however, would not be finally resolved until the Compromise of 1850.
Resolved by the Senate and House of Representatives of the United States in Congress assembled, That Congress doth consent the territory properly included within, and rightfully belonging to the Republic of Texas, may be erected into a new State, to be called the State of Texas, with a republican form of government, to be adopted by the people of said republic, by deputies in convention assembled, with the consent of the existing government, in order that the same may be admitted as one of the States of this Union.
2. And be it further resolved, That the foregoing consent of Congress is given upon the following conditions, and with the following guarantees, to wit:
First, Said State to be formed, subject to the adjustment by this government of all questions of boundary that may arise with other governments; and the constitution therof, with the proper evidence of its adoption by the people of said Republic of Texas, shall be transmitted to the President of the United States, to be laid before Congress for its final action, on or before the first day of January, one thousand eight hundred and forty-six.
Second, Said State, when admitted into the Union, after ceding to the United States, all public edifices, fortifications, barracks, ports and harbors, navy and navy-yards, docks, magazines, arms, armaments, and all other property and means pertaining to the public defence belonging to the said Republic of Texas, shall retain all the public funds, debts, taxes, and dues of every kind, which may belong to or be due and owning to said Republic of Texas; and shall also retain all the vacant and unappropriated lands lying within its limits, to be applied to the payment of the debts and liabilities of said Republic of Texas, and the residue of said lands, after discharging said debts and liabilities, to be disposed of as State may direct; but in no event are said debts and liabilities to become a charge upon the Government of the United States.
Third, New States, of convenient size, not exceeding four in number, in addition to said State of Texas, and having sufficient population, may hereafter, by the consent of the said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution. And as such States as may be formed out of that portion of said territory lying south of thirty-six degrees thirty minutes north latitude, commonly known as the Missouri compromise line, shall be admitted to the Union with or without slavery, as the people of each State asking permission may desire. And in such State or States as shall be formed north of said Missouri compromise line, slavery, or involuntary servitude, (except for crime) shall be prohibited.
3. And be it further resolved, That if the President of the United States shall in his judgement and discretion deem it most advisable, instead of proceeding to submit the foregoing resolution of the Republic of Texas, as an overture on the part of the United States for admission, to negotiate with the Republic; then,
Be it Resolved, That a State, to be formed out of the present Republic of Texas, with suitable extant and boundaries, and with two representatives in Congress, until the next appointment of representation, shall be admitted into the Union, by virtue of this act, on an equal footing with the existing States as soon as the terms and conditions of such admission, and the cession of the remaining Texian territory to the United States be agreed upon by the Governments of Texas and the United States: And that the sum of one hundred thousand dollars be, and the same is hereby, appropriated to defray the expenses of missions and negotiations, to agree upon the terms of said admission and cession, either by treaty to be submitted to the Senate, or by articles to be submitted to the two houses of Congress, as the President may direct.
Approved, March 1, 1845.
“That is urban myth. Upon statehood any territory becomes the same as any other state. The War of Northern Aggression cemented that relationship.”
Funny, King George also thought it was illegal for states (colonies) to secede.