If you mean who will provide protection, aircraft would probably be launched from mainland US. Supported by tankers, the US has no problem projecting air power

If you mean who will provide protection, aircraft would probably be launched from mainland US. Supported by tankers, the US has no problem projecting air power over all of Mexico, if allowed by the Mexican government (or even if not allowed, if they're willing to pay the diplomatic price). If you mean who would attack something like a high level international conference or sports events (with heads of major powers attending), that would obviously not be US carriers. (If the US navy or air force wants to bomb something, almost nobody can stop that.) Rather, the threat would be any terrorist or criminal group able to control either a civilian jet or large, fast drone (able to outfly the WW2-era fighters that Mexico still has available).