The Mexican and Civil Wars

Mexico declared the annexation an act of war. Fighting was triggered in May, 1846, by disagreement over the boundary, Texas claiming territory south to the Rio Grande and Mexico claiming all territory north to the Nueces River. The first battles between Mexican and United States troops were fought in the disputed area. Fighting then moved into Mexico; it ended with the fall of Mexico City late in 1847. In the peace treaty the Rio Grande was set as the southern boundary of Texas.

Early in 1861, over the opposition of Governor Sam Houston, Texas seceded from the Union and joined the Confederate States. Although little fighting took place within state boundaries, Texas served the Confederacy by handling trade with Mexico. After the Union victory in 1865, Texas was permitted to form a new state government. In 1867, however, Reconstruction leaders in Congress had the state placed under military rule, which lasted until 1870.