On June 23rd, 1845, The Republic of Texas joined the United States in a joint resolution vote of annexation. The Republic was formally annexed into the Union on December 29th, 1845.
Originally the leaders of the Republic of Texas attempted annexation with the United States in 1836 shortly after gaining independence from Mexico, but the United States was not interested in allowing another state to join the Union that permitted slavery within its borders.
Sam Houston, commander of the Texas Army during the battles with Mexico for independence, was an avid advocate of annexation with the United States.
Sam Houston was originally from Virginia, and came to Texas in 1832. He was a key leader in the Battle of San Jacinto, which was a determining battle that gave Texas its independence from Mexico.
The Republic of Texas originally extended all the way North into nearly the middle of modern-day Wyoming and included portions of Colorado, Kansas, Oklahoma and New Mexico.
Map of The Republic of Texas from wikipedia