Map of the Republic of Texas in green. The claimed area is in light green, while populated territory is in dark green.
The Republic of Texas was an independent and sovereign state since March 2, 1836 until February 19, 1846. The Texan colonists fought with US help to wean Texans territories of Mexico, in 1836. The new republic claimed boundaries that currently comprise the State of Texas, and the territories that now belong to the states of Colorado, Kansas, New Mexico, Oklahoma and Wyoming, based on the Treaties of Velasco between Texas and Mexico.
Texan flag. Texas was known popularly as the Lone Star Republic
Texan independence ushered in the short-lived Republic of Texas.
The Adams-Onis Treaty, defined the US border with Spain, since 1819. But the geographical boundaries of Texas with Mexico were never recognized by Mexico. Therefore, joining Texas to the United States, the problem of borders had to be defined after finishing a war. When states were unable border agreements, through diplomatic channels, they have to resort to use their military forces to define who owned what.