Mexico cannot control the issue.
That will require a fully militarized border and a 100 mile free-fire zone south of that border in which American Military forces can operate without restrictions.
The Government of Northern Mexico is the Cartels. They won't back down until they all dead. That is politically impossible right now.
A few more Cartel atrocities in the United States will change things.
Like I was saying. Laying the groundwork.