Is there any reason to believe that the Mexican government isn't bought and paid for by drug cartels?
Allowing for that possibility, then having the Mexican military running operations across the border doesn't sound outrageous.
I think it highly likely that there are numerous Mexican government officials who have been bought and paid for by the drug cartels.
The question is, though, on whom do we declare war in that case? If you're suggesting that this is the official policy of Mexico, then these incursions would be an act of war.
However, if the corrupt folks are implementing the "policy" of the drug lords, then the situation is quite a bit different.