Texas was annexed to the United States on Dec. 29, 1845. President John Tyler negotiated the handover, known as the Treaty of Annexation, in 1844. However, the treaty was not approved by Congress until the following year.
When the United States annexed Texas, Mexico cut diplomatic ties. Texas had previously been a province of Mexico before it gained its independence in 1836. Both sides disputed where Texas' border laid, with the Mexicans saying it was the Nueces River and with the Texans claiming territory that today comprises parts of modern-day Texas and parts of Colorado and New Mexico. This dispute would not be settled until the Treaty of Guadalupe-Hidalgo in 1848, which ended the Mexican War.