In the 76 days since President Biden imposed tough new restrictions on the nation’s asylum system, the number of migrants crossing the United States’ southern border illegally every day has dropped significantly.

Why?

The answer to that question is at the heart of a legal challenge to the president’s latest immigration policies, which critics say are undermining America’s decades-old role as a refuge for people fleeing violence, persecution, famine and economic dislocation.

The flow of migration across the hemisphere typically rises or falls because of many factors, including weather patterns, war, famine, economic conditions and immigration enforcement actions taken by other countries.

But decisions by the United States about how it polices the border also have an effect — including the asylum policy that a federal judge blocked this week.

Before the president’s asylum changes took effect on May 11, border patrol officials were encountering about 7,500 migrants trying to cross the border illegally each day — record-breaking numbers that were putting severe strains on the immigration officials and border communities.

Since then, the numbers have declined to about 3,000 migrants each day. That is still historically high, but dramatically lower.

On Tuesday, a federal judge ruled that Mr. Biden’s changes to the asylum system were illegal. Judge Jon S. Tigar of the U.S. District Court in Northern California said the presumption that most migrants crossing illegally are ineligible for asylum violates decades of law and, pending an appeal, “cannot remain in place.”

Administration officials warn that removing the new asylum restrictions could cause illegal crossings to spike again, though no one knows for sure what will happen.

Here is a look at the various forces at play when it comes to migration at the southern border.

Fear and Deterrence

The administration’s asylum policy has made it far less likely that a migrant fleeing from violence or persecution in Central or South America would be able to cross the border and remain in the United States while courts consider an asylum claim.

So that could be having a deterrent effect, causing some migrants in places like Venezuela, Nicaragua or Honduras to stay where they are rather than attempting a long, often dangerous journey to the southern border of the United States.

But immigration advocates say the policy is putting many migrants in danger by discouraging them from legitimately seeking refuge in the United States, effectively abandoning its traditional humanitarian role in the region.

Will the numbers of illegal border crossers increase if the judge’s ruling stands?

It’s possible.

Some migrants might decide it is once again worth the risk to travel to the U.S. border and claim asylum under the old rules. That could lead to a new surge of people heading north, especially if some are egged on by cartels and “coyotes,” who charge huge sums of money to help migrants make the journey to the border.

But as Judge Tigar noted in his ruling, ending Mr. Biden’s new policy would “restore a regulatory regime that was in place for decades before,” when the number of migrants crossing the border was far lower.

In communities along the border, migrants who have already made it into the United States were uncertain about what the judge’s ruling meant for them or people waiting on the Mexican side of the border.

Sitting in a bus station on Wednesday, not far from the Catholic Charities Respite Center in downtown McAllen, Texas, Herbin Moncada, 45, a native of Venezuela, scanned his phone to read about the latest news.

“Today they say one thing, and the next it changes,” Mr. Moncada said. “The truth is that you can’t trust what they say in the news. A judge issues a ruling, they go to court, fight it, and the next day, it is reversed.”

New Legal Pathways

When Mr. Biden imposed the new asylum policy in May, his administration also added new opportunities for some migrants — but not all — to enter the United States legally, without having to try to cross the border illegally.

The new opportunities are for migrants from four countries — Cuba, Haiti, Nicaragua and Venezuela — and are limited to a total of 30,000 people each month. Migrants can apply to be accepted while in their own countries; they do not have to travel to the U.S. border first.

Those who meet certain qualifications (like having a relative or friend to sponsor them in the United States) can be admitted for up to two years, with a work permit but no way to earn a permanent green card or American citizenship.

Administration officials have said the drop in illegal crossings at the southern border is in part the result of migrants taking advantage of the new opportunities. Officials have said that during the first half of 2023, nearly 160,000 migrants have come into the United States legally from the four countries. Illegal border crossings by migrants from the four countries has dropped 89 percent, officials said.

“The Biden-Harris administration’s commitment to expanding lawful pathways as an alternative to irregular migration has yielded positive results,” the Department of Homeland Security said in a recent statement.

But the new opportunities — known as “parole” — are not a solution for everyone, as Judge Tigar noted in his ruling, saying they will “necessarily be unavailable to many asylum seekers due to the parole programs’ limited scope and eligibility requirements.”

Smartphone Asylum

When the administration imposed the new asylum rules, they also expanded the ability for migrants at the border to make appointments to claim asylum at one of about 26 official ports of entry along the 2,000-mile border.

Those who make an appointment will not be subject to the tough new asylum rules. They will generally be allowed to wait in the United States while courts decide whether they are granted asylum — a process that sometimes takes years.

The catch? To do make an appointment, they must use a new smartphone app known as CBP One.

Administration officials say the new appointment system is helping to funnel some of the migrants away from illegal border crossings into a more orderly system at the ports of entry. Officials have said 30,000 migrants used the app to make appointments in May.

In downtown McAllen, two immigrants from Haiti, Fadeline Birote, 26, and Loodine LaBossiere, 29, said they applied using the app and even though it meant waiting longer to cross from Mexico, the process was more orderly.

“There are a lot of people waiting to cross,” Ms. Birote said. “Everybody is waiting for news, but it keeps changing. It’s very confusing.”

Immigration advocates have documented scores of technical problems with the government’s app that have prevented more migrants from successfully making appointments. And the number of appointments available daily at each port is limited.

Judge Tigar noted in his ruling that many migrants lack a smartphone and that those who wait in Northern Mexico for an appointment are often subjected to violence.

“Demand for appointments exceeds supply,” he wrote.

What Comes Next?

American border policies do not tell the whole story.

Apprehensions at the Mexico-U.S. border may be down in recent weeks. But further south, migration toward the United States has shown little sign of abating — an indication that many more migrants plan to show up at the border in the coming weeks.

The Darién Gap is the dangerous jungle land bridge that must be traversed to get from South America to the United States on foot. Once believed too dangerous to cross, in the past two years it has become a migrant thoroughfare, contributing to an unprecedented wave of South American migrants showing up at the U.S. border.

Last year, 248,000 people crossed the gap, a record that many officials in Colombia, Panama and the United States once thought inconceivable. This year, as of Monday, nearly 240,000 people had already crossed, according to migration authorities in Panama. (The jungle straddles Colombia and Panama.)

And down south, Mr. Biden’s policies seem to have had only a muted effect.

Roughly 40,000 migrants trekked through the Darién Gap in March, April and May each. In June, after Mr. Biden’s announcement of stricter asylum policies, that number dipped to 30,000. But by July it was back up — and higher — with almost 42,000 people crossing the jungle in the first 24 days of the month, according to Panamanian authorities.

Most of the migrants come from Venezuela, which has been in the grips of an economic, humanitarian and political crisis for nearly a decade. But a large number also come from Haiti and Ecuador, which are experiencing their own security crises.

People from China are the fourth-largest group in the jungle in 2023, with more than 10,000 crossing this year. Thousands of these migrants are children.