What did Donald Trump do today?

He told Mexico how to beat him in trade negotiations.

As is often the case, Trump spent the first part of the working day tweeting, and made this observation: "Mexico, whose laws on immigration are very tough, must stop people from going through Mexico and into the U.S. We may make this a condition of the new NAFTA Agreement."

In fact, Mexico already takes exceptional measures to guard its southern border in order to limit the number of people passing through to the United States. Mexico's "Programa Frontera Sur" was implemented under pressure from the Obama administration, and has intercepted hundreds of thousands of migrants bound for the United States. In fact, the program is generally regarded as far too effective: the military and police officials involved have been accused of human rights abuses.

Some Mexican leaders have suggested abandoning Frontera Sur in order to force Trump to the bargaining table on NAFTA. Trump's tweet is essentially admitting that this strategy would be effective, and that the United States would make concessions in order to keep what it already has.

Why is this a bad thing?

• Good negotiators don't tell their opponents how best to put pressure on them.

• American immigration policy cannot and should not be outsourced to Mexico.