On February 28, 1845, the U.S. Congress passed a bill that would authorize the United States to annex the Republic of Texas and on March 1 U.S. President John Tyler signed the bill. The legislation set the date for annexation for December 29 of the same year. On October 13 of the same year, a majority of voters in the Republic approved a proposed constitution that specifically endorsed slavery and the slave trade. This constitution was later accepted by the U.S. Congress, making Texas a U.S. state on the same day annexation took effect (therefore bypassing a territorial phase). One of the primary motivations for annexation was that the Texas government had incurred huge debts which the United States agreed to assume upon annexation. In the Compromise of 1850, in return for this assumption of $10 million of debt, a large portion of Texas-claimed territory, now parts of Colorado, Kansas, Oklahoma, New Mexico, and Wyoming, was ceded to the Federal government.

The annexation resolution has been the topic of some incorrect historical beliefs—chiefly, that the resolution was a treaty between sovereign states, and granted Texas the explicit right to secede from the Union. This was a right argued by some to be implicitly held by all states at the time, and until the conclusion of the Civil War. However, no such right was explicitly enumerated in the resolution. That having been said, the resolution did include two unique provisions: first, it gave the new state of Texas the right to divide itself into as many as five states (a proposal never seriously considered). Second, Texas did not have to surrender its public lands to the federal government. Thus the only lands owned by the federal government within Texas have actually been purchased by the government, and the vast oil discoveries on state lands have provided a major revenue flow for the state universities.