On Thursday, President Trump announced the imposition of progressively-mounting tariffs on imported Mexican goods. The tariffs begin at 5% on June 10; then they climb to 10% on July 1, 15% on August 1, 20% on September 1, and 25% on October 1.

The President said that the tariffs will be remain in place unless and until “Mexico substantially stops the illegal inflow of aliens coming through its territory.” What that means is open to debate; and it was probably intentionally left vague.

But there is no doubt what it should mean. Mexico must immediately sign a “safe third country” agreement with the United States, similar to our agreement with Canada. This would require asylum applicants to file their asylum application in the first safe country they set foot in (so applicants in the caravans from Central America would have to seek asylum in Mexico, rather than the United States).

In announcing the tariffs, President Trump suggested that Mexico should also start enforcing its own immigration laws against the migrants who illegally cross through Mexico en route to the United States. He is absolutely right. Mexico should arrest and deport those migrants back to their home countries in Central America, rather than simply encouraging them to continue north to the United States.

But that’s not enough. It doesn’t address the incentives that induce the Central Americans to illegally migrate to the United States in the first place. Perhaps the biggest incentive is the breakdown of our asylum system caused by the flood of illegal aliens who have no valid claim to asylum. As I have previously written, they come to the United States, make a bogus claim of asylum based on poverty or crime (rather than a valid claim based on persecution by their government), and enjoy a six-year delay in the United States until their asylum hearing.

A safe third country agreement would solve the problem. And it would be easy for Mexico to do, at little cost to them. Forced to make their asylum claim in Mexico rather than the United States, the vast majority of migrants would be unlikely to leave in the first place. Immigrating to the United States for economic reasons was their intention all along. And Mexico is just as likely to deny their asylum claim as the United States is.

Hopefully, this will be the first item on the table when the negotiations with Mexico commence.

If a safe third country agreement is not achieved, then crisis will likely continue. The numbers illustrate just how dire the situation is. May was the third month in a row that border apprehensions exceeded 100,000. And it was the highest monthly total in over twelve years. More than half a million have been apprehended since October 1, 2018.

To be sure, President Trump’s tariffs to compel Mexican cooperation are unprecedented. But so is the migrant crisis we are facing. It is unlike anything the United States has seen before. The President’s actions are exactly what is needed. Now we need him to hold firm until Mexico signs a safe third country agreement. Anything short of that would be insufficient to stem the flow of migrants.

Kris W. Kobach is the General Counsel of We Build the Wall. He served as the Secretary of State of Kansas during 2011-2019. An expert in immigration law and policy, he coauthored the Arizona SB-1070 immigration law and represented in federal court the 10 ICE agents who sued to stop Obama’s 2012 DACA executive amnesty. He also served as U.S. Attorney General John Ashcroft’s chief adviser on immigration law and border security.