There’s a very high likelihood that if the US wants to keep its territorial integrity then it will have to do so with blood and iron.
Unfortunately many of the current occupants of said territory are in favor of giving it back to Mexico.
It can also be accomplished with a few hundred microseconds of temperatures ten times hotter than the center of the sun.
Go back to 1910 to 1917 and you will find the Plan de San Diego. It’s goals were more than reacquiring the Mexican Cession but the extermination of white males in those states.
There's an extremely high likelihood that such lands would be defended by the people living there, likely with more effective fire. The military can take care of any hardware that accompanies the troops.
Oh, and yeah, I'm absolutely sure that there'll be a hell of a lot of people with Mexican ancestors defending our country from the corrupt Mexican government.