Look at Mexico. For decades, this was a country defined by fiery anti-Americanism. Founded by a radical revolutionary movement, fueled by anger against American imperialism and high-handedness, Mexico would rarely cooperate with Washington. Since the 1990s, the landscape has shifted, indeed almost reversed. Thanks to intelligent leadership in Mexico City and consistent bipartisan engagement by Washington, the United States and Mexico have become friendly neighbors, active trading partners, and allies in national security.

Mexico buys more U.S. goods than does China and is, in fact, the second-largest destination for U.S. exports after Canada. Sales to Mexico are up 455 percent since the passage of NAFTA. The country cooperates with the U.S. on border security, helping to interdict drug shipments and deporting tens of thousands of Central American migrants who aim to enter the U.S. illegally. Mexico is an ally of the U.S. in most international negotiations and organizations.

All of this could change easily. Over the last year, as candidate Trump and now President Trump has attacked and demeaned Mexico and its people, the political landscape there has shifted. President Enrique Pena Nieto’s already-declining approval ratings have plummeted after he was seen as too conciliatory toward Trump. It is now quite possible -- in fact, likely -- that the next president of Mexico will be an anti-American socialist-populist similar to Venezuela’s Hugo Chavez. Andres Manuel Lopez Obrador was polling around 10 percent at the start of 2015. He is now around 30 percent, the front-runner among the potential candidates for next year’s election. 

A victory for Lopez Obrador would be a disaster for Mexico -- but also for the United States. It would likely take Mexico back to its days of corrupt socialism and dysfunctional economics, all sustained by populism and nationalism. Lopez Obrador has described Trump as a “neo-fascist,” attacked the Pena Nieto administration for being too weak to confront Trump, and promised to get tough with Washington. In February, he began a tour of several American cities, speaking to large rallies of Mexican-Americans and symbolically standing up to Donald Trump. 

Now consider South Korea. Trump’s demand that Seoul pay for the THAAD missile defense system, threatening to overturn the existing agreement with Washington, has fueled the forces in South Korea that oppose that system in the first place, along with any aggressive military measures against North Korea. Trump has casually delivered a number of slights to one of America’s closest allies, accepting wholesale China’s claim that Korea once belonged to it and threatening to tear up the U.S.-South Korea free trade agreement. South Korea is facing a snap election for its presidency, and the candidate who is benefiting most from Trump’s antics is the left-wing Moon Jae-in. Anti-Americanism has returned to South Korea in force, though not quite as strongly as in Mexico, where Donald Trump’s favorability has been recorded at 3 percent.

Were these trend lines to harden, it could mean decades of difficulty for American foreign policy. Dealing with North Korea is hard enough as it is, but with a recalcitrant South Korea that is determined not to be viewed as overly pro-American, it would become impossible. Tackling issues of drugs, border control and migration would become much harder if the Mexican government recoiled from cooperating with the United States. 

There are other places where the Trump effect is also clear. Politics in Iran have become more favorable to hard-liners, and the re-election of the relatively moderate President Hassan Rouhani, once seemingly assured, is now in jeopardy. Supreme Leader Ali Khamenei appears to be campaigning against him and supporting a far more anti-American candidate. In Cuba, Raul Castro has gone from inching toward better relations with the U.S. to lambasting Trump and his policies. In every country in the world, America’s friends are embarrassed and on the defensive, and its enemies are gloating. 

In foreign policy, great statesmen always keep in mind one crucial reality -- every country has its own domestic politics. Crude rhetoric, outlandish demands, poorly thought-through policies and cheap shots all place foreign leaders in a box. They can’t be perceived as surrendering to America, and certainly not to an America led by someone who is determined to show that for America to win, others must lose. That’s one big difference, among many, between doing a real estate deal and managing foreign policy.