Final answer:
Texas entered the United States as the 28th state in the year 1845, after having been an independent nation as the Republic of Texas from 1836.
Step-by-step explanation:
The Annexation of Texas to the United States
Following the Texas Revolution and the establishment of the Republic of Texas as an independent nation in 1836, the region operated under its own governance with Sam Houston as its first president. After several years of negotiation and political maneuvering, Texas entered the United States as the 28th state in 1845. The annexation was a reflection of the era's 'Manifest Destiny' philosophy and the southern expansionists' interests, especially those pertaining to the expansion of slavery. Despite initial hesitations due to the possible outbreak of war with Mexico and heightened internal disputes over slavery, Texas was admitted into the Union leading to increased tensions that would contribute to the outbreak of the Mexican-American War.