LONDON >> President Donald Trump’s announcement of sweeping restrictions on travel from much of Europe to the United States dropped like a hammer on Wednesday.

He said they were an attempt to stem the spread of the coronavirus. But the announcement left Americans and Europeans alike confused about when the restrictions come into effect, how they will be enforced and whether they will have the intended effect of halting the spread of the coronavirus.

Here’s what we know.

The restrictions apply only to the 26 nations in the Schengen Area.

After Trump addressed the nation from the Oval Office, leaving the impression that all of Europe would be subject to the travel restrictions, the government scrambled to clarify.

The restrictions apply only to people who are not U.S. citizens who in the past two weeks have been in European nations that are part of the Schengen Area — the 26 countries that generally allow free and open movement across their borders: Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherland, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden and Switzerland.

Limiting the restrictions to the Schengen Area allows some notable exceptions across Europe, including Britain, Croatia, Cyprus, Ireland, Turkey, and Ukraine.

In one instant, Trump referred to a “foreign virus” and criticized European nations as having “failed to take the same precautions” as he had in limiting travel from China, where the outbreak began.

>> Click here to see our full coverage of the coronavirus outbreak.

Europe has not been a major source of known infections in the United States, but Italy has been hit hard, reporting more than 12,000 cases. France, Germany and Spain have each reported more than 2,000 cases.

The White House said the rationale for limiting travel from the Schengen zone was because, combined, they have the most confirmed coronavirus cases outside China — 17,442 cases and 711 deaths as of Wednesday — and showed “high continuous growth in infection rates.”

“The free flow of people between the Schengen Area countries makes the task of managing the spread of the virus difficult,” the White House said in a statement, adding that travel “threatens the security of our transportation system and infrastructure and the national security.”

The restrictions begin at 11:59 p.m. Friday, Eastern time.

By midnight Friday, the new restrictions will be in full force.

But they will not apply to anyone aboard a flight to the United States that left before that time.

Hours after the announcement, it was still unclear how flights that operate regularly between Europe and the United States would be affected, and airlines were scrambling to create contingency plans.

American Airlines said that customers holding U.S. passports or Green Cards attempting to re-enter the United States from Europe would be rebooked, if necessary, to one of the approved airports. But the carrier added that any other passengers who had traveled to one of the Schengen countries within the past two weeks would not be allowed to travel to the United States on its flights.

Air France said it would announce today whether it would adjust flights as a result of the restrictions. But Norwegian Airlines, a low-cost carrier with several routes between Europe and the United States, said its flights would continue as normal in the short term.

Not all foreign travelers will be subjected to the travel restrictions. The spouses, parents or siblings of American citizens or permanent residents, will be exempt, as will members of the U.S. military and their spouses and children.

For Americans in Europe, it just got more complicated.

Permanent residents of the United States will not subjected to the restrictions.

U.S. citizens who have traveled in the specified European nations will still be able to enter the United States. But the practicalities could become increasingly complicated.

Trump noted in his address that American citizens who had traveled in the Schengen Area could enter the United States once they had been “appropriately screened.”

Vice President Mike Pence said on the television show “Fox & Friends” today that Americans returning from Europe in the next 30 days would be “funneled through 13 airports” and asked to self-quarantine for 14 days. He did not specify which airports.

Pence said the measures would remain in place for 30 days.

The European Union apparently was not told what was coming.

The European Union, which seems to have been caught off guard, today slammed Trump’s decision.

Ursula von der Leyen, the president of the European Commission, and Charles Michel, the president of the European Council, said in a scathing joint statement that the decision was made “unilaterally and without consultation.”

It is unclear whether European officials will cooperate with the travel restrictions.

Muriel Pénicaud, the French labor minister, sought to reassure the French public, but said that the restrictions would be a blow to the transportation and export sectors.

She added, however: “It will harm the U.S. too. Trade goes both ways.”

Jean-Baptiste Djebbari, France’s junior minister for transportation, said today that he was not surprised by Trump’s decision but suggested that France had not received any warning.

“I know the temperament of Donald Trump, who by the way maybe reacted a bit later than some other countries, most notably European countries, on the issue,” Djebbari told reporters in Paris after a meeting with representatives from France’s transportation and travel businesses.

The announcement unleashed chaos at airports.

Travelers eager to get to the United States before the restrictions took effect described scenes of chaos and uncertainty at airports across Europe today.

At Charles de Gaulle Airport near Paris, long lines began forming at dawn, some passengers resigned to waiting hours with no guarantee of a ticket.

A ticket agent for Delta said some panicked passengers had gone online in the hours after Trump’s announcement and spent tens of thousands of dollars for last-minute flights to the United States. They then tried to change them for cheaper tickets at the airport, without success.

At the Barcelona Airport, Carlos Martinez, a 26-year-old from San Francisco, and his boyfriend struggled to find an affordable ticket home after canceling the second leg of their European trip.

“We would like some clear direction from our government as to what is the protocol here,” Martinez said in a phone interview.

© 2020 The New York Times Company