Political map of North America & the Caribbean 29 December 1845 (Annexation of Texas): An even bigger flashpoint than Oregon (Oregon dispute) was Texas, which wanted to join the US despite Mexican warnings that doing so would be considered an act of war. The first US attempts to annex Texas were blocked by Congress, with free state (Slave and free states) politicians worried about Texas' slave state status. Nonetheless popular opinion in both countries strongly favored annexation and Texas was finally admitted to the Union in 1845 (Texas annexation).