Timothy Henderson's A Glorious Defeat: Mexico and its War with the United States (2007) is a thoroughly enjoyable account of the events leading up to, and through, the 1846-1848 war. It is written for a general audience, and he does a great job of explaining complex situations in a really engaging manner.
The complexity lies in bringing together the different strands of Mexican political turmoil (and the ever-present Antonio López de Santa Anna), the politics of Texas, and the politics of the United States. The U.S. was moving westward and believed Mexico both incapable and not chosen by God to have what became the western United States. Texas was looking for autonomy from Mexico, became independent, and then annexed to the U.S. Mexicans watched in disgust, but were too crippled by corruption, factions, and a ragged conscript army to resist. Finally, the U.S. fabricated a provocation in order to declare war.
I found the analysis of Mexican politics to be very compelling, as it acknowledges how Mexican politicians failed to create the sort of political institutions that would keep the country together, even as they faced Americans who disdained them and had no compunction about lying and stealing land. The most fascinating individual has to be Santa Anna:
Politics, for Santa Anna, seems to have been primarily the art of finding the right despot, and for him that search usually ended satisfactorily only with himself (p. 80).
He would rule, be exiled, and yet time and time again, Mexican political elites called him back. Along those lines, my favorite passage from the book may be the following:
Moderate politician José Fernando Ramírez, who was normally of a very skeptical turn of mind, wrote to a friend, "There is no doubt whatsoever that [Santa Anna] is returning as a real democrat, and I can conceive of his being one." Ramírez, perhaps realizing how absurd that sounded, added laconically, "I cannot tell you on what I base my conviction." (pp. 159-160).
Santa Anna left in disgrace after Chapultepec Castle was taken and U.S. troops marched into Mexico City (though he would return yet again later!). For anyone who has not been to Chapultepec Park, there is a massive monument to six teenage cadets who died rather than surrender. Here is a photo I took of it when I was in Mexico City last month:
Mexico finally agreed to terms, in large part because the vacuum caused by the war led to indigenous uprisings, including 200,000 dead in the Yucatán Peninsula in 1848 alone. Many elites wanted to bring in European monarchy to rule, though they would not be successful in that endeavor for more than a decade.
Indeed, one critical conclusion that tends to get less attention is the following:
The territorial transfer unleashed toxic political passions that a little over a decade later would plunge both the United States and Mexico into bloody civil wars. The U.S.-Mexican War exacerbated the imbalance of power and wealth that had, in large measure, caused it in the first place, and that imbalance has yet to be corrected (pp. 179-180).