MEXICO CITY (NYTIMES) - President Donald Trump's plan to cut off aid to three Central American countries for failing to stop the flow of migrants towards the United States breaks with years of conventional wisdom in Washington that the best way to halt migration is to attack its root causes.

The decision also runs counter to the approach advocated by Mexican President Andres Manuel Lopez Obrador, among others. Mr Lopez Obrador has been lobbying Washington to join his government in investing billions of dollars in Central America and southern Mexico, arguing that economic development and reducing violence are the most effective ways to encourage Central Americans to remain home.

Cutting off aid is "shooting yourself in the foot", said Ms Adriana Beltran, director of citizen security at the Washington Office on Latin America, a human rights research group that tracks aid closely.

But Mr Trump has become incensed at the growing number of families arriving at the US border with Mexico asking for asylum. His administration notified Congress late last Friday (March 29) that it intends to reprogramme US$450 million (S$615 million) in aid to Guatemala, Honduras and El Salvador, and has already sent instructions to embassies in the region.

"No money goes there anymore," he told reporters last Friday. "We're giving them tremendous aid. We stopped payment."

While legislators have tools to push back against that decision, it is very possible that some, if not all, of that aid could be suspended for now.

The decision turns US policy in the region on its head. Not only will it cut development and humanitarian assistance, but it will also halt joint law enforcement efforts, such as anti-gang units vetted by the US, that had been supported by Republicans and the Trump administration until now, said Mr Juan Gonzalez, a former deputy assistant secretary of state in the Obama administration.

Indeed, just a day before Mr Trump made the comments, the US signed a border security agreement with the three Central American governments intended to increase cooperation against human trafficking and organised crime.

Mr Gonzalez said the aid withdrawal "undermines our interest", adding that "we have actually had success against gangs in the United States by cooperating with regional law enforcement. It helped us prevent increased gang flow".

The decision also caught Mexico off guard. The government there was already rattled by Mr Trump's threat to close parts or all of the border as early as this week in response to the immigration surge, and this was an added blow.

Advocates argue that stopping aid will only aggravate the root causes that drive migrants to leave the three countries, where a long history of corrupt governments and rigid inequities perpetuate deep poverty.

Gang violence, drug trafficking and abusive security forces - some of it the result of US policies in the region that focused on fighting communism in the 1980s and drug trafficking since the 1990s - have led to the highest homicide rates in the world outside of war zones.

The Obama administration ramped up aid after a surge of Central American children arrived at the Texas border in 2014. Aid to the region doubled in 2016 to about US$750 million, according to the Washington Office on Latin America.

Ms Beltran said aid after 2016 not only focused on violence and insecurity but also reflected an understanding that "you needed to address the issues of governance and corruption, and you needed to create economic opportunities and build institutions". With significant aid reaching the region only in 2017, there has not been much time for it have a strong impact.

"There are long-term challenges that are going to need a long-term sustainable solution," Ms Beltran said. "You can have a discussion as to how we can ensure that the aid is effective, that assistance is not going to supporting corrupt governments."

Much of the humanitarian aid is distributed through local governments and non-governmental organisations. Cutting off that help is "illogical and vindictive", said Mr Tim Rieser, a senior foreign policy aide to Senator Patrick Leahy, vice-chairman of the Senate Appropriations Committee.

But cutting off direct aid to the national governments of the Northern Triangle countries may be long overdue, he added, because they are part of the problem.

"Senator Leahy does not believe we should support governments that care more about enriching themselves and staying in power than addressing the needs of their own people," Mr Rieser added, pointing to efforts by the governments of Honduras and Guatemala to control the courts and thwart anti-corruption efforts.

The Trump administration has lifted some of the pressure as the governments of Guatemala and Honduras cultivated conservative allies in Washington and presented themselves as allies in drug interdiction.

To win favour with Washington, Guatemala followed the Trump administration in moving its embassy in Israel from Tel Aviv to Jerusalem in 2018. Honduran President Juan Orlando Hernandez said last week that his government was opening a trade office in Jerusalem, which he called "a first step" towards moving his country's embassy.

There was no official response from Central American governments on Saturday. Mr Ebal Diaz, minister of the presidency of Honduras, told Radio America, a Honduran broadcaster, that US aid was largely directed to non-governmental humanitarian and aid groups.