what had the United States done that made Mexico believe that they might try to make Texas a part of the United States?

Texas was annexed by the United States in 1845 and became the 28th state. Until 1836, Texas had been part of Mexico, but in that year a group of settlers from the United States who lived in Mexican Texas declared independence ... The annexation of Texas contributed to the coming of the Mexican-American War (1846-1848).