From California to Texas there are thousands of miles of open border. There may or may not be assets in place to deal with this threat. Areas that cannot be guarded should be rendered unusable. Trenches could be dug, cement filled barrels could be strategically placed to thwart takeoffs and landings. Remote areas could be mined.
Helicopters pose an almost indefensible threat.
This is where the old civil defense air watch could be reintroduced. Radar can be avoided by low level flight. But a person can often see and hear what high technology misses. We do not hear much from our friends from south of the border. They seem to do little or nothing about drug trafficking or illegal immigration.