Oh man, this is gonna be big! Saw it on Wingnut friend's FB page

[politicat]

92. That territory was acquired via the Treaty of Guadalupe Hidalgo in 1848 from Mexico.

There were multiple things going on (various wars, land claims) but one of the reasons for the specific purchase to include what became the Utah Territory was to bring the Deseret colony under some effective control, since having a theocratic kingdom (that was going through its own fairly bloody Reformation) with a history of armed combat and counterfeiting on the western or northern border was about as attractive then as now.

Neither Mexico nor the US really wanted to take on the task, but the US was slightly better equipped to do so (the crossing from St Louis to SLC is tough, as anyone who has driven across I80 can attest, but it's an infinitely easier crossing than from Mexico City to SLC. The south to north route has both a distinct lack of water and navigable water ways. The east to west has somewhat more surface water, several more or less navigable rivers, and while the Continential Divide is no picnic, it's slightly less spectacularly fatal than running out of water in the Mojave.) Some of the subtext of the writings around that treaty make it sound like the diplomatic negotiations considered the colony to be more concession than prize, or "fine, you can have Colorado and California for $15M, but you have to take the desert and the Mormons, too."