Texas had fought for its independence against Mexico in 1836, but ran into problems with maintaining their status as an independent state. Since most of the Texans were from the United States frontier, joining the U.S. was kind of like a no-brainer for preventing a reconquest by Mexico. The U.S. finally annexed Texas in 1845, and agreed to pay $10 million in debt, and almost immediately provoked the Mexican-American war. Oh and all this is what gave Texas the power to be able to split itself into four smaller states if needed.

Texas annexation 1845