The government must keep the borders porous, the drugs illegal, and the mayhem going in order to maintain the pretense of their own necessity. If they actually solved problems they would put themselves out of a job... They could again be a part time legislature...

Follow the money. Eliminate the border to make it easier on the cartels. Create a 'crisis', then declare marshal law to 'protect' the citizens. Easier to protect people if they are in a concentrated area so force them out of the suburbs by gas taxes they can't pay, then use all the ammo DHS has amassed to 'protect'.