The same thing that prevents any enemy from sending a thousand guys to any other sensitive site - men with guns (and intel and air support and armor).
Occasionally the cartel and Texas guard do exchange fire.
Occasionally the cartel and Texas guard do exchange fire.
This isn’t something the government hopes doesn’t happen: it’s something they actively work hard at everyday to ensure doesn’t happen.
A thousand man force is a battalion. The cartels have no real armor and no air support. No heavy weapons. No training.
The US Army fields 31 brigades. A brigade is 5 battalions.
For every cartel member in your theoretical force, there are ~149 trained, armed, active duty service members in deployable condition for combat roles.
That doesn't include artillery, sustainment, the US Airforce or the US Navy. I am also not including guard teams which are another 27 brigades.
Your theoretical cartel force would also have to go through the Texas State Guard (1,700 people) and US Border Patrol.