The Republic of Texas was established in 1836, after the Texas Revolution (1835-1836) between Mexico and Texas colonists gave the latter independence.
Fearing that Mexico would try to re-establish its oppressive rule on the Republic, Texans were very much in favor of being annexed by the United states from as early as 1936, but the proposal was rejected by both the Andrew Jackson and Martin van Buren Administrations.
Britain was also opposed to the idea, as it wanted to block the United States from expanding further west. However, this attitude actually helped sway Americans toward annexation.
Thus on December 29, the proposal of the Republic was approved by the US Congress and Texas became the 28th state of the Union.