Frankly, I'm not even entirely sure why we made it. According to Wikipedia it was about the Union-Pacific and Jefferson Davis' dreams of territorial acquisition while Secy. of War. To this day about all we have to show for $10 million is Tuscon.

As a Texan, I know full well why the US went to war with Mexico, and while Santa Anna is much to blame (had he upheld the 1824 Constitution as he was sworn to do TX might still be part of Mexico, and the terms of OUR treaty required him to forever renounce the Presidency. ) What's a lot more hazy is why that war, its casus belli or its outcome justify the Mexican Cession. What that boils down to is the US Army occupying all of Mexico, and allowing Mexico proper to regain its independence after extracting a hefty pound of flesh (probably more than anything because our territory was expanding TOO quickly at the time, forcing a showdown between "traditionalists" who wanted, on one hand, to restrict slavery to its historical limits and, on the other hand, to preserve the historic balance of power between agricultural slave and industrial free states. ) We screwed the natives more than adequately after we took control though; where's Geronimos skull? Don't tell the Comanche, Apache or Navajo that the US was blameless in the Mexican Cession (to be fair, troubles with the first two date back to the acquisition of TX, but most of TX is outside the easternmost range for the Comanches during their dominance. )