The supreme court has given the Trump administration a green light to block asylum seekers at the US-Mexico border, in a decision that fundamentally reshapes the US asylum system.

The decision allows the Trump administration to revive its so-called turn-back or “metering” policy, allowing federal agents at the US border to stop migrants from physically setting foot on US soil, where federal law guarantees them the right to claim asylum and protection from persecution.

The vote was 6-3, with Justices Samuel Alito, John Roberts, Clarence Thomas, Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett concurring. Justices Elena Kagan, Ketanji Brown Jackson and Sonia Sotomayor dissented, with the latter penning a biting 35-page long dissent – notably almost twice as long as the Alito majority opinion.

Because US immigration law entitles migrants arriving in the US to seek asylum, the supreme court case hinged on what, exactly, it means to “arrive in”.

In Alito’s opinion, he wrote: “In ordinary speech, no one would say that a person ‘arrives in’ a place … before the person enters that place.”

Sotomayor pushed back strongly in her dissent: “The court’s illogical interpretation is driven almost entirely by a fixation on a single word: ‘in’. Words, however, must be read in context and with attention to how they fit into the statute as a whole.”

The decision, she said, would have dire consequences – allowing the government to circumvent laws protecting asylum seekers by blocking their entry at the border.

“They may do so even if the asylum seeker is at the threshold of a port of entry designated to receive all non-citizens who seek entrance into the country. Even if the port of entry has ample capacity to inspect that person, including an available asylum officer trained to process asylum applications. Even if the asylum seeker is certain to be persecuted, or killed, if she is turned away,” she wrote.

Human rights advocates have said that the court’s decision allows the Trump administration to essentially invalidate international and US asylum laws, which require government officials to inspect people arriving at ports of entry and ensure that they are not being turned back to dangerous conditions.

The decision is the culmination of a legal battle that has spanned three presidential administrations. The case was originally filed in 2017, during the first Trump administration, by Al Otro Lado, a legal and humanitarian service provider based in California and Mexico, and a group of asylum seekers subjected to the turn-back policy.

Rather than allowing migrants to turn themselves in at ports of entry and immediately evaluating their eligibility for asylum, officials stood at the border line between the United States and Mexico to block their entry to the US. Migrants were stranded at dangerous encampments or in temporary housing in Mexico while they entered their names on a list to await the opportunity to apply for asylum.

Depending on where migrants were along the border, these lists were kept in physical notebooks or in digital format, maintained by Mexican immigration officials or municipal authorities. Confusion and a humanitarian crisis ensued, and some migrants turned away at official ports attempted dangerous crossings across the Rio Grande or the Sonoran desert.

Joe Biden rescinded the policy in 2021, but after Donald Trump was re-elected to the presidency, his administration asked the supreme court justices to review lower court rulings banning the practice.

Thursday’s decision will have far-reaching consequences, said Erika Pinheiro, Al Otro Lado’s executive director. “In a world of increasing conflict and climate disaster, this hardening of borders to keep out the most vulnerable is sure to result in many more lives lost,” she said.

The metering policy is currently not in effect at the border, but if it is reimplemented, advocates said they expect another humanitarian crisis will ensue.

Rebecca Cassler, a senior litigation attorney at the American Immigration Council, added: “Cruelty is not a substitute for real solutions. Blocking people from seeking asylum at official ports of entry will do nothing to fix our broken immigration system; it only makes things more chaotic and dangerous for vulnerable families.”

The supreme court decision came after lower courts repeatedly invalidated the practice of turning away asylum seekers at the border. Advocates and attorneys representing migrants have argued that the practice is illegal and contrary to the country’s long history of providing refuge to those fleeing persecution.

But the Trump administration has long seen asylum as a roadblock in its goal of shutting down the southern border. Last year, administration officials promoted a global campaign to roll back asylum protections, seeking to dismantle the post-second world war framework supporting refugees and asylum seekers. During a United Nations gathering in September, the deputy secretary of state, Christopher Landau, characterized the asylum system as “a huge loophole in our migration laws”.

The US Department of Homeland Security (DHS) under Trump has sought to not only turn away people arriving at the border, but also urged immigration courts to summarily dismiss asylum claims. Increasingly, the DHS has been sending migrants fleeing persecution in their home countries to third countries where they have never been.

A shift toward limiting access to asylum at the southern border, however, began during the Obama administration, when officials sought to “meter” the flow of migrants into the US. Asylum claims at the border had increased alongside dwindling opportunities for other types of immigration, and lengthy backlogs for visas and green cards. In 2016, tens of thousands of people from Haiti began arriving at the southern border seeking safety; many had first sought to settle in Brazil after the catastrophic 2010 earthquake, but made their way north after Brazil, too, experienced a stark economic downtown.

That’s when immigration agents, in some cases, began standing at international bridges, seeking to prevent migrants from reaching ports of entry to the US.

Though much of the supreme court oral arguments about the case centered on the question about what it means to “arrive” in the US and whether the administration is entitled to deny asylum by preventing arrival, the court’s liberal justices also grappled with what it would mean to essentially end the practice of providing asylum at the US border.

Justice Sotomayor compared the practice of turning away asylum seekers to the tragedy of the St Louis, a passenger ship with Jewish refugees that was turned away from the US right before the second world war. About half the passengers who were returned to western European countries were trapped and killed when Germany invaded.