The Republic of Texas was supposed to be annexed in October 1845 as a Treaty of Annexation was the requirement in order to annex a sovereign nation.

Texas became a state in December 1845 after statehood was granted by the U.S. Government, however, after research at the national archives of the United States and the state Capital of Texas, no Treaty of Annexation has ever been found and no vote by the people of Texas for Annexation was ever held.

There ya go!