For all the uncertainties in U.S. foreign policy, one thing is clear: U.S. President Donald Trump loves tariffs. He is convinced that they are the most valuable negotiating tool he has, singularly capable of focusing the attention of adversaries and bending them to his will. Trump believes that trade wars, fought primarily with tariffs, are "easy to win" since the U.S. economy is so much bigger than any other and the pain it can inflict is invariably greater than that it will suffer.

That guiding philosophy was ostensibly confirmed last week, after the United States and Mexico announced a new agreement to curb illegal migration into the U.S. Reportedly infuriated by news that more than 130,000 people coming from Mexico had been caught by U.S. border agents in May, the highest monthly level since 2006, Trump warned that the U.S. would impose 5 percent tariffs on all Mexican goods — with a threat that the tariffs would eventually rise up to 25 percent — if Mexico did not do more to control the flow of people across its southern border. That triggered three days of negotiations that produced a deal in which Mexico agreed to take "strong measures" to cut those numbers.

While Trump administration officials hailed the agreement as a breakthrough, the deal was initially criticized for merely repeating pledges by Mexico to take action. Mexican government documents noted that the deal had broken new ground, however: It was "the first time in recent history that Mexico has decided to take operational control of its southern border as a priority." The agreement is a shift by the Mexico government, which last year argued that migrant enforcement by Mexico was "dirty work" being done for the U.S.