He doesn't need to.
The border can be policed by satellites and a few thousand low-cost drones. The technology is cheap. It is available now.
These are cheap drones that carry no missiles. They only carry cameras. There can be night drones and day drones. They can observe what is going on for miles.
Put up 1,000 drones at all times, day and night. Aim their cameras 10 or 20 miles south of the border. Is there a truck coming north toward a desolate area? Meet it with troops.
There will be enough troops on the cheap fenced border to head off 80% of the trucks. If 80% of the trucks can't get their cargo through, the wholesale immigrant business is finished.
Post a thousand troops and drones at the high-crossing areas.
The strategy here is to eliminate the wholesale immigration business. Make the immigrants walk to the border in remote areas. This will drastically reduce the crossings. Move the immigrants from wholesale to retail.
MAKE A DEAL WITH THE COYOTES
Coyotes are the people who take money from desperate would-be illegal immigrants to take them across the border in trucks. They are middlemen.
I propose that Trump make them two-way middlemen.
A coyote brings a truckload of people to the border. On the American side of the border is a border patrol officer who is assigned to an Army unit. He makes the truck driver an offer: "Take them back. Here is $25,000, cash."
Yes, I know: pretty soon there will be trucks loaded with fake immigrants. So, run the initial program for long enough to get the word out: "Don't pay anyone to take you across the border. The gringos have cooked up a deal with the coyotes. The coyotes will take your money. Then they will take the gringos' money to bring you right back." Six months will do it.
If traffic starts back up, re-start the program.
Keep the immigrants on the Mexican side of the border. This way, there will be no expensive court hearings on our side of the border. There will be no ACLU attorney coming to the defense of illegal immigrants in the middle of the night in the South Texas desert. The immigrants will not cross the border. Mexico will retain jurisdiction.
Station enough troops and drones on the border to seal it off.
Better yet, arrange for the coyotes to bring the would-be immigrants to established check points. No muss, no fuss.
But, you say, no sensible coyote would do this. It would kill his business. So, stage it. Hire some Mexican Americans in South Texas to do a fake crossing at a fake border. Rent an old truck. Have an actor pay the "driver" a wad of bills. Use a dash cam. Make it look real. Then post the video. Do one of these a week. Use the same two or three trucks.
Have you ever seen Wag the Dog?
Trump can supply the video crew. He is a master of reality TV.
Keep the drones flying. Let the coyotes know that they will not be able to get their cargoes across the border. "We can see you coming."
Maybe a truck turns around and heads home. The driver does not get $25,000. Word will get out: "Don't pay this guy."
Maybe the driver stops. He gets his $25,000. He drives them back. Word will get out. "Don't pay this guy."
Maybe he unloads them on his side of the border. He gets $25,000. He leaves them. Word will get out. "Don't pay this guy."
If he crosses the border, he gets no money. He loses his truck. He gets no food or water. He is sent back on foot with the others. "Have a nice walk with your friends."
The people who get unloaded will be given two choices: (1) put on a GPS tracking device and start walking back to civilization with food and water supplied by the Americans, or else (2) don't put on the device and get no food or water. There is this threat: "Take off the device before you get to town A, and the next time we catch you trying to cross illegally, you will get no food and water. And we will catch you. Take the device to our local exchange store, and they will give you $100 cash." Of course, they are photographed at the border. Their faces go into a facial recognition data base. If they get caught again, they get no food or water for the hike back. "Buy food and water from your buddy."
Why the exchange store? The store will advertise locally. "If you get caught when our drone system catches the truck that takes you to the border, and if the driver leaves you behind, you can get $100 for the GPS device you will get when you return from the border. We'll make sure you are in our 'caught trying to cross the border' facial recognition system. Then you will get paid."
Word will get out to the coyotes' customer base.
Post Spanish language videos on YouTube. The viewers will see the drones, the troops, and the money exchanges. "Warning: the U.S. government has an arrangement with the coyotes. It pays the coyotes to bring people right back at a profit of $25,000 per truckload. You will pay $1,000 for a one-day round-trip visit to the border. The coyotes will make money both ways."
Put up videos of families walking away from the fence in the middle of the desert. "This is what happens if you try to cross the border in a truck. The drones always spot the trucks. You will be greeted by American troops. Save your money."
Goodbye, wholesale immigrant market. Hello, individual families on foot.
BETA TEST THIS BY LATE APRIL
This would be cheap to implement. The beta-testing could be operational within three months of Trump's inauguration.
Put a bird colonel in charge of this. Offer him two stars if he gets it operational by Christmas, but only one star if it's a day late. Trump can make this offer on January 21. He can ask for volunteers. He will get lots of volunteers by January 23. By February 1, there will be piles of applications.
This colonel would be granted the authortity of a four-star general for this project until December 25, 2017.
The colonel will find holes in the system. Most of them can be plugged. He could then ramp up the program by October 31. It would be working by December 25.
Give the program until December 31, 2018 to work. By then, the flow of illegal aliens will be down by 80%.
Priority for Army promotions would be given to successful captains and majors who cut down immigration rates. The statistics would be simple: "Reduce the immigrants in your sector by 20% this year, and you go to the top five of the short list for promotion." The Mexican border would be the fastest track to stars on the shoulder.
Word would get out. Fast.
CONCLUSIONS
Could Trump do this unilaterally? Of course. He is Commander-in-Chief. Paul Ryan isn't. Mitch McConnell isn't.
Trump doesn't need a wall. He needs only the will.