TAM,
It was in the US law that annexed Texas to the Union, not in in Constitution. That's why I say it was a
limitation on
Texas. Think how many states you could make out of Texas if they were all the size of Delaware.
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(Of course there has to be some reasonable population to go with them.)
Recall this was 1845/46, when the slavery issue was heating up. Apparently some were concerned that if a territory as large as Texas was annexed it could subsequently be split into many states -- perhaps swinging the balance of power between slave and non-slave states. (However, the US's impetus for annexing Texas was not pro- or anti-slavery, but that the British were sniffing around in the west, trying to limit US expansion). So part of the joint resolution of congress that authorized the annexation of Texas limited the number of (additional) states that could be formed to four, with ones below the "Missouri compromise line" able to choose to be free or slave states, and ones north of the line to be free states only.
The text:
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 said state, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution. And 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 into the Union with or without slavery, as the people of each state asking admission may desire. And in such state or states as shall be formed out of said territory north of said Missouri compromise line, slavery, or involuntary servitude, (except for crime,) shall be prohibited.
The entire joint resolution is here:
http://avalon.law.yale.edu/19th_century/texan01.asp" onclick="window.open(this.href);return false;