WASHINGTON — President Trump’s detractors are trying to play down the significance of the U.S.-Mexico immigration deal, saying it is largely comprised of actions that Mexico already had agreed to many months ago.

Nice try. If Mexico had truly agreed to implement many of these measures in December, then why had they not been implemented six months later? As even Mexican officials acknowledge, it was Trump’s threat of tariffs that forced Mexico’s hand. In announcing the deal, a relieved Mexican Foreign Minister Marcelo Ebrard said the “most important thing is that they didn’t apply tariffs and we didn’t experience an economic slowdown.” The fact is, Trump bucked Republicans on Capitol Hill and even many of his own advisers, and used the threat of tariffs to get Mexico to act — and it worked.

The measures Mexico has promised to avoid those tariffs include the deployment of 6,000 national guard forces at Mexico’s largely unguarded border with Guatemala, which — according to Mexican negotiating documents — will be “the first time in recent history that Mexico has decided to take operational control of its southern border as a priority.”

Mexico also agreed to carry out thousands more arrests of illegal migrants each week and to remove the caps it imposed on the number of U.S. asylum seekers it would accept and hold inside Mexico.

Ultimately, the Trump administration wants Central American migrants to have to seek asylum in Mexico rather than the United States. The United States and Canada have a “safe third country” agreement under which those seeking asylum must make their claim in the country where they first arrive. If Mexico were designated “a safe third country,” those crossing its southern border would have to seek asylum and settle there. That would push the Central American migrants problem from our southern border to Mexico’s southern border where it belongs.

Mexican officials convinced Trump to delay his demand for a “safe third country” agreement and give them 45 days to show that the other measures in the agreement will work. If they don’t, foreign minister Ebrard understands that Mexico will either have to accept new asylum measures or face tariffs.

The president deserves credit for forcing a reluctant Mexican government to act. He was able to do so because the administration in Mexico City knew he was willing to pull the tariff trigger.

Everyone advised him not to do it, but Trump saw his leverage and he used it. Mexico now has 90 days to show progress — and so does Congress. If members do not like the blunt instrument of tariffs, then they should give the president some alternatives. Otherwise, they will be unable to avoid blame if a trade war with Mexico ensues.

Marc Thiessen writes a twice-weekly column for The Post on foreign and domestic policy and contributes to the PostPartisan blog. He is a fellow at the American Enterprise Institute, and the former chief speechwriter for President George W. Bush.