In short the US ends up keeping it. Slavery would never have been allowed into Mexico in the same way it wasn't allowed into California or anywhere else west of Texas. The war on the ground becomes unconventional. the US army's cavalry arm is expanded rapidly. And a large number of Mexicans are recruited by the occupying army(this happened in OTL).
If the ACW breaks out in 1850 or 1860 is a big deal.
In 1850 the southern economy is stronger but secessionists are a lot weaker in the upper south. Southern cavalry is also more effective without the Minie ball and rifled muskets. The southerners however face a much larger Union army. The south is however cut off from its main artery for supplies(In OTL most supplies came through Mexico)
In 1860 the southern economy is weaker but pro-secessionists are a lot stronger in the upper south. With the responsibility of policing Mexican waters and supporting the US army, the US navy is a lot stronger than OTL at this time. Technology has blunted the Confederacy's main advantage of horse cavalry.
If the ACW breaks out in 1850 or 1860 is a big deal.
In 1850 the southern economy is stronger but secessionists are a lot weaker in the upper south. Southern cavalry is also more effective without the Minie ball and rifled muskets. The southerners however face a much larger Union army. The south is however cut off from its main artery for supplies(In OTL most supplies came through Mexico)
In 1860 the southern economy is weaker but pro-secessionists are a lot stronger in the upper south. With the responsibility of policing Mexican waters and supporting the US army, the US navy is a lot stronger than OTL at this time. Technology has blunted the Confederacy's main advantage of horse cavalry.