The Supreme Court has partially reinstated Donald Trump's travel ban - meaning that citizens from six Muslim-majority countries without any connection to the US will now be barred from travelling.
Mr Trump said last week that the ban would go into effect 72 hours after receiving an approval from the courts.
Two federal appeals courts have previously blocked critical parts of the order, meaning that Monday's ruling is therefore a partial victory for Mr Trump, who wanted all citizens from Somalia, Sudan, Yemen, Iran, Syria and Libya barred from arriving on US shores.
He described the decision as "a clear victory for our national security."
He continued: "As president, I cannot allow people into our country who want to do us harm. I want people who can love the United States and all of its citizens, and who will be hardworking and productive.
"My number one responsibility as commander in chief is to keep the American people safe. Today's ruling allows me to use an important tool for protecting our nation's homeland."
The Supreme Court stopped short of banning all arrivals, however, as Mr Trump had wanted.
But it did agree to an emergency measure, taking effect immediately, which will prevent anyone who cannot prove a solid connection with anyone already in the US from arriving.
"The interest in national security is an urgent objective of the highest order," they said in their ruling.
Only those "with a credible claim of a bona fide relationship with a person or entity in the United States," can pass US borders.
It means that students and those with family in the US can still travel to the country - although it did not specify what family ties and what "entities" were acceptable.
Justice Clarence Thomas, joined by Justices Samuel Alito and Neil Gorsuch - appointed by Mr Trump - dissented from part of the court’s opinion, and argued that they would have revived the travel ban in its entirety while the court considered the case.
Justice Thomas noted that the ruling was difficult to interpret.
“I fear that the court’s remedy will prove unworkable,” he wrote.
“Today’s compromise will burden executive officials with the task of deciding — on peril of contempt — whether individuals from the six affected nations who wish to enter the United States have a sufficient connection to a person or entity in this country.
“The compromise also will invite a flood of litigation until this case is finally resolved on the merits, as parties and courts struggle to determine what exactly constitutes a ‘bona fide relationship,’ who precisely has a ‘credible claim’ to that relationship, and whether the claimed relationship was formed ‘simply to avoid’ the executive order."
The justices' decision granted parts of his administration's emergency request to put the March 6 executive order into effect immediately while the legal battle continues.
The court also said it would partly allow a 120-day ban on all refugees entering the United States to go into effect.
David Miliband, president and CEO of the International Rescue Committee, condemned the decision, pointing out that refugees already undergo intense screening and background checks, which often take 36 months.
“Too much time already has been spent litigating this misguided order,” he said.
“The approach of the administration is bad policy. That is not changed by the legal arguments.
"The Court’s decision threatens damage to vulnerable people waiting to come to the US: people with urgent medical conditions blocked, innocent people left adrift, all of whom have been extensively vetted.
"We urge the administration to begin its long-delayed review of the vetting process and restart a program which changes lives for the better.”