Delayed by five months of legal challenges before the Supreme Court partially backed Trump, the ban comes into effect at 8 pm Thursday Eastern time (0000 GMT Friday), putting tight restrictions on the issuance of visas to travelers from Iran, Libya, Somalia, Sudan, Syria and Yemen.
Officials stressed that anyone with a valid visa issued before that time would still be admitted, promising to avoid the airport chaos that accompanied the original travel ban announcement in January.
All authorized refugees booked for travel before July 6 will also be admitted.
"We expect business as usual at the ports of entry starting at 8 pm tonight," said a senior administration official. "Our people are well prepared for this."
Nevertheless immigration activists and lawyers said they would be at airports to support any arrivals unfairly denied entry.
"The world is watching the United States of America, and what they are saying is, we thought that it was the country for opportunity and justice for all, but it does not seem that way," said Murad Awawdeh of the New York Immigration Coalition, speaking at New York's John F. Kennedy International Airport.
Define 'bona fide'
But questions remained over the Supreme Court's decision Monday to allow exemptions for anyone having a "bona fide relationship" in the United States.
According to guidelines issued in a State Department cable to embassies, that exemption will include people with "close family relationships" in the United States, defined to include parents, spouses, children, sons and daughters-in-law, siblings, and step- and half-siblings.
But "close family" does not include grandparents, grandchildren, aunts, uncles, nieces, nephews, cousins, brothers-in-laws and sisters-in-law, fiances and any other "extended" family members, the guidelines say.
People with formal relationships with a US entity, who have for instance been offered a job or accepted to study or lecture at a university, will also qualify for visas during the ban. But a hotel reservation, even if already paid for, does not qualify.
And the order stresses that non-profit groups cannot establish relationships with hopeful travelers or refugees just to allow them to skirt the ban.
90 day halt
Trump first ordered the travel ban in January, following up on campaign promises to secure US borders against the threat of terrorism.
It set a 90-day halt on travelers from the six countries, plus Iraq, while the government reviewed and toughened its vetting procedures. It also set a 120-day ban on all refugee entries.
But implementation was immediately frozen in federal courts based on complaints that the president had overreached his executive powers and violated the Constitution by essentially focusing his order on Muslims.
A revised ban -- which notably removed Iraq from the country list -- was also blocked in March, forcing the administration to appeal it to the Supreme Court.
This week the top court's justices agreed to hear the case in October, and in the meantime partly lifted the lower courts' freeze orders in a win for Trump's administration.
Lawyers and advocates both for and against the travel ban say the decision could result in a flood of legal challenges.
Lawyers at the ready
Gregory Chen, Director of Government Relations at the American Immigration Lawyers Association, said the State Department guidelines are fairly straightforward but very tightly defined.
They don't account for the amount of contact someone has with a relative, for example, someone raised by a family member not their parent.
"If you had frequent contact with your aunt, that should be indicative of a close family relationship .... The cable does not seem to take that into account."
Rama Issa, executive director of the Arab American Association of New York, said the government is redefining what a family is.
"I was raised by my grandparents so the idea of grandparents not being part of a family is very foreign to me," he said.
"I'm engaged to get married. I have family who lives in Syria today, not only my father, but my aunts and uncles who I would love to be at this wedding, and unfortunately are not going to be able to be here."
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