The whole aregument of "It used to be ours" is stupid.
Mexico was a Spanish Territory until 1810. Mexico became a country in 1821. Then, Mexico lost the Texas war for Independance in 1836. So Texas was "Mexico's" only for 15 years. The Texas settlers were fine with being part of Mexico, until Santa Anna declared himself dictator, like Napolean.
The Country of Texas and the borders between Texas and Mexico were not entirely settled after the war. And large tracks of what is now New Mexico and other states were part of Texas.
There was a movement in Chihuahua to become part of the United States, about the same time Texas was looking for admisssion.
After Texas joined the Union, the US offered to buy the land West and North of El Paso, and set the border where it is today. This lead to the war with Mexico, Treaty of Guadalupe Hidalgo and payment of about 1/2 what was previously offered.
So, the land of New Mexico, Arizona and California was bought and paid for with blood and gold. It is ours, like it or lump it. If you want it...
