“Put the US troops on the Mexican side of the border by agreement with Mexico.”
Whether they agree or not.
The Mexican government does not control its territory and will probably be overthrown by the cartels in a few years.
Destroying the cartels with extreme prejudice would be a critical part of securing our borders from invasion. At 12:00:01 EST on Inauguration Day, all 5 branches of the US Military could/would/should launch a coordinated attack on every single cartel known to the US, wiping them off the face of the earth.
It would also send a message to our enemies elsewhere that there is a seismic shift in US policy to include robust protection of our national sovereignty and defense.