At least two of the major candidates for the US Presidency and a few members of Congress (including a Senator) have suggested unilaterally sending the US military to take out the cartels [1] [2] [3].
It would be a years-long occupation with no clear end goal.
It would kill at _least_ hundreds of thousands and negatively affect the lives of an order of magnitude more.
It would almost certainly fail by any reasonable metric. Look at how occupying Iraq and Afganistan went, or how the US has done in South America over the last century.
Yeah and cartel membership isn't so simple. Most people in any cartel prominent state know cartel members and have a complicated relationship with it and them. They don't want some foreign military coming in and killing their uncle/cousin. They want better opportunities for their family members who are affiliated. In fact - that would probably be much cheaper than trying to kill cartel members with drone strikes.
I live in Mexico and talk to people here about the clowns that are running for president in the US. It's genuinely embarrassing.
Edit to address actual reasons it would be bad:
1) Mexicans (in Mexico) distrust the US and its military. The US should not damage the remaining trust it has with an enormous trade ally. That would push Mexico towards China.
2) Could the US significantly damage the cartel(s) on their own turf?
3) What are the cartels? Are they the literal president-level politicians who are affiliated? Should the US "take them out" too?
I could go on but really the short answer is that the US would end up causing a mess, ruining the lives of normal people, and damaging its relationship with a major world economy and major trade ally.
They should call Iraq and Afghanistan for references /s
It's an impossible situation, but I don't think it's difficult to understand why Mexicans would not want the US military to invade based on its track record.
The US military couldn’t capture one man, Poncho Villa, the last time they entered Mexico and they pissed off most of the country in the process. How are they going to round up more than 100k while not overstaying their welcome?
Also I think the US military is not the best at fighting an insurgency.
The US military cannot do this effectively because our rules of engagement prevent collateral damage. The Mexicans have to do what El Salvador did, build a massive prison out in the middle of nowhere and essentially pick people up off the straight based on suspicion and ship them off. It has to be sudden and everywhere all at once.
Coincidentally, this started right after they found a gigantic source of lithium and said the state would control the mining of it, blocking American companies from exploiting it.