"America First is a fallacy,” President Andrés Manuel López Obrador, often referred to as AMLO, said in a public letter released late Thursday.
López Obrador also announced he was sending his foreign minister to Washington, D.C., on Friday to negotiate with U.S. officials ahead of a June 10 deadline set by Trump.
In a Thursday tweet, Trump said that on June 10 the United States "will impose a 5% Tariff on all goods coming into our Country from Mexico, until such time as illegal migrants coming through Mexico, and into our Country, STOP."
In the tweet and in a separate written statement, Trump claimed that the tariff will escalate by 5 percentage points each month until "the Illegal Immigration problem is remedied," at which time it will be removed. Trump said the tariff could reach 25% by Oct. 1.
As for Trump's statements, AMLO said: “With all due respect, although you have the right to express it, 'America First' is a fallacy because until the end of times, even beyond national borders, justice and universal fraternity will prevail."
While not citing the letter specifically, Trump did defend his proposal in Friday morning tweets, blaming Mexico and congressional Democrats for migrant problems on the border.
"Mexico has taken advantage of the United States for decades," Trump tweeted, claiming that "Mexico makes a FORTUNE from the U.S." and "they can easily fix this problem."
In another tweet, Trump claimed that companies would leave Mexico to avoid the tariffs, but it shouldn't come to that. "Mexico must take back their country from the drug lords and cartels," he said in a tweet. "The Tariff is about stopping drugs as well as illegals!"
Trump did not define what a sufficient solution would be to stop the wave after wave of Central American migrants journeying through Mexico to reach the U.S. southern border.
Through the first seven months of the 2019 fiscal year, U.S. Border Patrol agents have apprehended more than 280,000 migrants entering the country illegally who are minors or members of family units. Of those, 97% are from El Salvador, Guatemala and Honduras, and only 3% are from Mexico, according to Border Patrol data.
Get the Behind The Storynewsletter in your inbox.
A behind-the-news look interview about how awesome USA TODAY NETWORK stories come together, delivered every other Monday.
In his written statement, Trump also said: "Workers who come to our country through the legal admissions process, including those working on farms, ranches, and in other businesses, will be allowed easy passage."
In his public letter responding to Trump, López Obrador said Mexico is already doing what it can to stop the migration, and added that “social problems are not solved with duties or coercive measures."
He said he is developing plans with Central America and told Trump: "Please, remember that I do not lack valor, that I am not a coward nor timorous but rather act according to principles.”
Citing the United States' history as a nation of immigrants, López Obrador also told Trump: "The Statue of Liberty is not an empty symbol.”