Mr. Trump himself mentioned this bilateral cooperation as part of the trade deal. At a White House news conference on Oct. 1, when flaunting the agreement, he said it explicitly: “Yeah, we talked about it (the wall, border security). With Mexico, we talked about it. It was a big part. And certain things and certain understandings are had. At the same time, we don’t want to mix it up too much. This is a very big deal and very good deal for everybody. But border security and security generally is a very big factor.”

He was probably referring to the two main American demands on Mexico that have been made public in the United States press over the past weeks. They are ominous for Mexico and ignoble for the United States.

First, Washington has been pressing the outgoing president, Enrique Peña Nieto, to sign what is known as a Safe Third Country Agreement with the United States. This entails Mexico processing Central American asylum claims locally, and allowing American authorities to deny any claim for asylum in the United States, since Mexico would be considered a “safe third country.” With violence in Mexico at its highest point in decades, and many Central Americans being murdered there in recent years, it is hard to see the merits of such a certification by Washington, much less its acceptance by Mexico. So far, Mr. Peña Nieto has resisted that pressure; the caravan is moving freely through Mexico.

One can suppose, however, that the incoming president, Andrés Manuel López Obrador, may have less leeway, especially if this arrangement was a precondition for a new NAFTA. Mr. Trump has stated that the Honduran caravan members must request asylum first in Mexico without any legal basis for doing so. Mexico has no reason for proceeding this way, except for United States pressure.

Second, and most important, since 2014 under President Obama, and now more than ever, Washington has pressured Mexico to shut down as much as possible of its southern border — in a nutshell, to do its dirty work. Mr. Peña Nieto has reluctantly gone along, but the cost for Mexico is rising, and he is soon to be no longer in charge.

This is why it’s reasonable to assume that the Trump administration conditioned its acceptance of a diluted new NAFTA on Mexico’s acceptance of more cooperation, or complicity, with the United States, for keeping Central Americans, be they refugees, economic migrants, or a combination of both, from reaching the United States border.

This is what Mr. Pompeo said in Mexico: Stop the Hondurans from reaching the United States border. Mr. López Obrador has insinuated that he won’t maintain this policy. Knowing him on and off for more than 30 years, I think the chances are that if he is left no choice, having agreed to the NAFTA/immigration trade-off, he might cave in to American pressure, despite his displeasure and current denials. He has staked too much on USMCA as a source of economic stability for his incoming government to put it at risk simply because of Central American refugees. But deporting thousands of Central Americans or placing them by force in refugee camps would be intolerable for his supporters.