Some things to keep in mind about this ever-popular “division of Texas” issue:
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The Republic of Texas, during its nine years of existence, claimed all territory between the Rio Grande and the pre-Mexican War boundary between the United States and Mexico. This is an enormous swath of territory including much of what is now New Mexico, Kansas, and Colorado. The boundary had never been surveyed, nor agreed to by Mexico, and much of the territory still belonged to American Indians.
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In March 1845 Congress annexed this territory to the United States, together with an “enabling act” allowing it to organize itself into a state. Everybody recognized that creating such an enormous state might cause problems:
a. Pro-slavery forces realized that, by creating an enormous slave state, they would be hurting their chances for creating more slave states later.
b. Anti-slavery forces recognized that part of the Texas claim probably lay north of the 36.5-degree line of the Missouri compromise, meaning that its inclusion in Texas would extend slavery north of the line.
c. And just as a matter of simple governance, after this territory was settled, mamaging it all as one giant state might probe problematic.
Rather than resolve these issues in 1845, Congress just passed the “enabling act” to annex and admit Texas as a whole, but added the proviso about later division into as many as four more states.
The phrase “shall be entitled to admission under the provisions of the Federal Constitution” causes confusion here. It sounds binding. But remember, no Congress can bind a future Congress. Rather, the words should be viewed as a statement of “legislative intent” for the guidance of a future Congress which might have to consider the issue. In other words, if the Congress of 1855 were considering an application for division, they could look at the wording of the resolution of annexation and see at once that the Congress of 1845 didn’t regard the Republic of Texas as an inviolate whole and recognized that division might one day be appropriate.
As it happend, Congress stripped Texas down to its present boundaries in return for a cash settlement as part of the Compromise of 1850. So, in effect, the original Republic of Texas did divide into more states; but they were remanded to territorial status first and agglomerated with other land won in the Mexican War.
As a non-binding statement of legislative intent, however, the language of 1845 remains in effect. It wouldn’t carry much weight today–if the remainder of Texas did seek to divide into two or more states today, I doubt that anybody would much care what Congress thought under the very different circumstances of 1845.