My great-great grandfather, Spencer B. Lawson was a private in the USArmy under Winfield Scott during the Mexican-American war of 1846-1848. The United States drove all the way into Mexico City, thus defeating Santa Anna. The Americans then negotiated a peace treaty with the Mexican government. This Treaty of Guadalupe Hidalgo ceded a major portion of Mexican territory to the conqueror, actually comprising about 55% of the land area of the country.*
The cession gave all of California, Nevada, Utah and Texas to the United States, as well as some or most of the states of Arizona, New Mexico, Colorado, Oklahoma, Kansas and Wyoming.
The accompanying map shows what this land mass looks like relative to the current state boundaries.
The hostilities actually ceased on February 2, 1848 and a treaty was subsequently hammered out. The treaty was ratified by the United States Senate on March 10, and became of full effect on May 19 when it was ratified by Mexico.
There are so many "what ifs" surrounding this whole era of history with regard to Mexican-American relations that it would take volumes just to delineate them, to say nothing of analyzing the effects even to now, and into the future.
My favorite "what if" is What if we had subjugated Mexico, took over the entire country to the borders with Guatemala and Belize and annexed it into the United States?
An equally valid "what if" is What if the cession had never occured and the entire "American Southwest" were still part of Mexico?
And finally, What if old guys with too much time on their hands did not engage in the futile activity of imagining suppositions regarding the altering of the past?
*This map includes Texas as part of the cession, which is not technically accurate, in that The Republic of Texas was established in 1836 and annexed as a US state in 1845. Mexico, however never relinquished its claim to the lands of Texas. Since the war and treaty discussed here ultimately resolved that dispute, I have included all the land that the United States acquired from Mexico, as well as the disputed area known as Texas. The southern-most portions of Arizona and New Mexico shown in brown on the map were later acquired from Mexico through the Gadsden Purchase of 1853.