My understanding over the US/MX cartel relations is performing an invasion and “act of war” would solidify asylum status claims by Mexican residents and throw a wrench into the whole immigration scheme every administration plays.
But then again this time seems different, laws aren’t followed or upheld. Human rights are a fleeting staple.
Starting a war with Mexico would be a pretext for interning everyone of "Mexican" ethnicity, citizen or otherwise, as was done to Japanese nationals.