In 1836, the Republic of Texas voted to be annexed by the US. Initially, when the Texas minister in Washington, D.C, proposed annexation to the administration of Martin Van Buren in August 1837, the request was refused since the administration was afraid of wars with Mexico and Texas withdrew the annexation offer in 1838. In 1843, Great Britain opposed annexation, and President John Tyler, who did not want to do anything to help Great Britain, proposed annexation. Despite the fact that Mexican leader Santa Anna warned that annexation would be "equivalent to a declaration of war against the Mexican Republic," John Tyler signed a treaty of annexation with Texas in April 1844. Sam Houston consented to the proposed annexation and Tyler signed a treaty of annexation with Texas in April 1844, but the United States Senate rejected the treaty in June 1844. A factor, always in the background, during the Texas Annexation discussions in the United States was the fear of the north that the slave states would gain representation when Texas was admitted as a slave state.
After James Polk, a strong supporter of territorial expansion, won the presidency, but before he took office, Congress approved the annexation on 28 February, 1845. On 29 December, 1845, Texas ceased being an independent nation and now has representation as a state as defined by the U.S. Constitution.
Both the Joint Resolution for Annexing Texas and The Ordinance of Annexation contains this language providing the basis for forming up to four additional states from the present Texas:
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;