Most of the land west of the Missippi river used to be a possession of Spain before some of it briefly passed through France's hands on its way to ours (Lousiana Purchase) (The Oregon Territory had been a joint US/British possession, but prior to that several different countries had been trying to claim it, including Spain.). California, Nevada, Utah, a good chunk of Arizona and some of New Mexico was handed over to the United States in the Treaty of Guadalupe Hidalgo. And there's that 'infamous' Gadsden purchase, in whose area I've been living for the past twenty years. This area was wanted to build a southern transcontinental railroad, (remember that locomotive I showed you earlier?). So our president sent James Gadsden (who had personal interests in the rail route) to negotiate the Gadsden Purchase with Mexican President Antonio López de Santa Anna.

It was originally going to be a larger area, but there were all kinds of objections to this. "Even the small strip of land that was ultimately acquired was enough to anger the Mexican people, who saw Santa Anna's actions as yet another betrayal of their country and watched in dismay as he squandered the funds generated by the Purchase. The Gadsden Purchase helped to end Santa Anna's political career."(from Wiki)

At first this newly enlarged area was all just 'New Mexico territory'. Then it was divided into Arizona and New Mexico territory, and in 1866 Arizona and New Mexico took their now familiar boundaries. On a old map of 1867, you can see Tucson is there. But Yuma has a different name. Yuma was first known as Colorado City until 1858, when it was called Arizona City. Not until 1873 was Yuma given get its present name by the Territorial Legislature. Furthermore, it wasn't until 1912 that each of the two territories were granted statehood.