The White House has choked off Cuba’s oil supply and threatened a “friendly takeover” of the communist-run island, against a backdrop of military operations in Venezuela and Iran.

U.S. President Donald Trump is implying the country is his next target, saying: “Whether I free it, take it,  I think I can do anything I want with it. They’re a very weakened nation right now.” The oil shortage is bringing Cuba’s economy to the brink. But I’ve found myself thinking back when, not that long ago, it briefly looked like the two nations would normalize relations after decades of hostility.

I first landed in Havana in March 2012 to cover Pope Benedict XVI’s visit. The airport was small. I had to repeatedly explain to immigration officials that we were there as journalists, that we had permission, and that everything had been cleared in advance. I was grateful that my team spoke Spanish to help with the process.

Parts of the city felt strangely familiar from images I’d seen of faded pastel buildings and old American cars somehow still running on patched-together parts.

Cuba and the U.S. had been geopolitical foes for more than 50 years. Cuba became communist when the 1959 revolution brought Fidel Castro to power and the island nation, just 90 miles from Florida, strengthened its ties with the Soviet Union. The Cuban government seized U.S. property and American-owned businesses in response to a growing U.S embargo. In response, President John F. Kennedy formalized a full embargo in 1962. Supplies of food, fuel, and consumer goods quickly became scarce.

But being there, I sensed that something was beginning to shift.

Between 2012 and 2016, I made 10 trips, field producing for CNBC with international correspondent Michelle Caruso-Cabrera. Almost every visit seemed to line up with something significant — moments that felt like they might mark a turning point. But by the end, that momentum felt suddenly uncertain.

On my first visit, Havana was trying to look ready for a pope. Fresh paint lined parts of the Malecón, still drying in places along the route the pope was expected to travel. In a country shaped for decades by communism, his presence felt like more than a religious event. It felt like a signal, subtle but unmistakable, that Cuba might be opening up.

After that, things started to move quickly.

Less than a year later, the government invited a small group of journalists, including us, to see what it called “reforms” up close. We spoke with the central bank governor, and with small business owners trying to navigate a system that was changing, but not all at once.

We slipped away from the official itinerary and made our way to Hershey, Cuba, a town Milton Hershey built to secure sugar for his chocolate business in the early 20th century. It was one of several reminders of Cuba’s American past before its revolution. A former Coca-Cola factory had been repurposed by the state. A Western Union building housed the country’s telecom company. A Woolworth’s store had become a local discount store.

In July 2015, President Barack Obama announced the restoration of diplomatic ties. We moved quickly, out of New York, down to Miami, then onto a charter flight to Havana. On the ground, there was a real sense of excitement. But it wasn’t unguarded. People were hopeful, but careful.

A month later, the U.S. embassy reopened for the first time in more than 50 years. I watched the flag go up from the balcony of a crumbling apartment building across the street. For younger Cubans especially, it felt like a turning point: More opportunities, more access, more choice seemed within reach.

Obama’s visit the following March only added to that feeling. Travel restrictions for Americans were relaxed and limited trade began to restart. The embargo was still in place, as it is written into U.S. law, but it did slightly soften.

That week brought a Rolling Stones concert and a Major League Baseball game, the first on the island in years.

Even then, there was restraint. Cubans had learned not to get ahead of themselves. For many, optimism came with the memory of how quickly it could fade. After all, not everyone believed the United States should reopen relations with the country. Many argued that normalizing ties would reward the communist government without forcing meaningful reforms.

Still, things were changing. In 2016, Carnival Cruise Line, under its Fathom brand, docked in Havana, the first U.S. cruise ship to visit the island since 1978. By November, JetBlue had direct flights running from New York. For a time, it felt like the barriers were coming down in real time.

Reporting there was never simple. Permits could fall through without warning. Phones rarely worked. Wi-Fi was hard to find. Restaurants handed out long menus, but when you asked, you were often told the only thing available was rice and beans. I’d walk past buildings with elegant facades, only to step inside and find them hollowed out, crumbling, little more than dust and debris.

And yet, on each trip, you could see small signs that the transformation was continuing. Family-run restaurants began opening in people’s homes. Airbnb listings started to spread. It wasn’t dramatic, but it was there.

My final trip came in November 2016, just after Fidel Castro’s death, to cover his funeral. He’d ceded power to his brother Raoul years earlier, but the death of the man who symbolized the revolution was a huge moment.

This time, Havana was quiet.

Music stopped. Alcohol disappeared. The city entered a formal mourning period. People stood in long lines to sign condolence books.

From the outside, it looked like a clear ending. Inside Cuba, it didn’t feel that simple.

Standing there, it was hard not to feel that the energy of the previous years was slipping away. The same questions kept coming back. What happens now? What becomes of the reforms? Of the relationship with the United States?

When I left for the last time, I had the sense I’d witnessed something rare, a brief stretch of time when history seemed to accelerate, when long-standing patterns loosened, even if only slightly, and the future felt, for a moment, open.

In the years since, much of that momentum has slowed, and in some cases reversed. The U.S. withdrew embassy personnel, new travel limits were imposed in November 2017, and the flow of American visitors thinned. The opening that once felt within reach has given way to more familiar tensions, which are flaring like the changes I saw never happened.

History doesn’t always arrive with a clear beginning or a clean ending. In Cuba, it has a tendency to circle back on itself.

What comes next between these two neighbors is still unwritten.

Airline and travel stocks fell Monday after airspace closures throughout the Middle East forced carriers to cancel thousands of flights, disrupting trips as far as Brazil and the Philippines.

United Airlines, which has the most international exposure of the U.S. carriers, was down 6% in premarket trading. Service to Tel Aviv, Israel, is one of the airline’s most profitable routes, but airlines were also was forced to pause flights to Dubai, in the United Arab Emirates, one of the busiest airport hubs in the world.

Dubai is a home base for airline Emirates.

Shares of Delta Air Lines and American Airlines were also each off about 6%. Flights through the Middle East were grounded including to destinations like Tel Aviv.

Other carriers like Southwest Airlines, which is more U.S.-focused, had smaller stock moves but shares still fell as investors assessed a possible run-up in oil prices. Fuel is generally airlines’ biggest cost after labor.

Hotel chains also fell, with Marriott International and Hilton Worldwide Holdings down.

International travel has been a bright spot in the travel sector. In January, international air travel demand jumped 5.9% from a year ago while domestic flight demand was nearly flat, the International Air Transport Association, an airline industry group, said in a report on Monday.

Travelers are stranded as far away as Australia, Brazil and the Maldives after the U.S. and Israel launched strikes on Iran this weekend. With airspace in the region still closed, getting home could be a challenge at least several days.

Here’s what to know:

The Transportation Security Administration said on Sunday that its PreCheck airport screening lanes are operational, an about-face hours after the Department of Homeland Security said the faster security checkpoints were paused amid the partial government shutdown.

Travel industry leaders said they received little, if any, warning of the changes to PreCheck, a program that allows its 20 million pre-screened members to pass through airport security faster than at standard lanes. Industry members spoke with DHS officials in the past few hours and expressed alarm about the sudden decision, people familiar with the matter said.

“At this time, TSA PreCheck remains operational with no change for the traveling public,” TSA officials said in a statement. “As staffing constraints arise, TSA will evaluate on a case by case basis and adjust operations accordingly. Courtesy escorts, such as those for Members of Congress, have been suspended to allow officers to focus on the mission of securing America’s skies.”

DHS early Sunday said that PreCheck and Global Entry and other program suspensions were scheduled to take effect at 6 a.m. ET on Sunday. As of 12:40 p.m. ET, its updated statement still included a suspension of Global Entry but it had removed its mention of PreCheck.

“We are glad that DHS has decided to keep PreCheck operational and avoid a crisis of its own making,” Geoff Freeman, chief executive of U.S. Travel, an industry group whose members include major airlines, hotel chains like Hyatt and Marriott International and tourism boards around the country.

The move comes as a partial U.S. government shutdown that has left thousands of DHS workers, including TSA airport screeners, working without pay since it started on Feb. 14.

“TSA and CBP are prioritizing the general traveling population at our airports and ports of entry and suspending courtesy and special privilege escorts,” DHS Secretary Kristi Noem said in a statement.

Noem blamed Democrats for the shutdown.

“Shutdowns have real world consequences, not just for the men and women of DHS and their families who go without a paycheck, but it endangers our national security,” she said. “The American people depend on this department every day, and we are making tough but necessary workforce and resource decisions to mitigate the damage inflicted by these politicians.” 

Senate Minority Leader Chuck Schumer, (D-N.Y.), pushed back, saying the Trump administration is “choosing to inflict pain on the public instead of adopting common sense” reforms of Immigration and Customs Enforcement, or ICE.

DHS did not say whether it expected to reverse its suspension of Global Entry or what prompted the change. The White House referred an inquiry from CNBC to DHS.

Travel industry experts sharply criticized the move before it was reversed, which comes just months after last year’s record federal government shutdown cost airlines millions of dollars and hurt bookings, according to executives. The sector’s leaders have repeatedly complained about how air travel has ended up at the center of repeated shutdowns and have pushed lawmakers to ensure that essential government workers are paid during funding lapses.

The government shutdown in the fall, the longest ever, cost the travel industry and other sectors $6.1 billion, the group said. Those disruptions affected about 6 million travelers.

“A4A is deeply concerned that TSA PreCheck and Global Entry programs are being suspended and that the traveling public will be, once again, used as a political football amid another government shutdown,” said Airlines for America CEO Chris Sununu. The group represents American Airlines, Delta Air Lines, Southwest Airlines, United Airlines and other major carriers.

“The announcement was issued with extremely short notice to travelers, giving them little time to plan accordingly, which is especially troubling at this time of record air travel,” he added.

The U.S. Travel Association said earlier: “We are disgusted that over the last 90 days, Democrats and Republicans have used air traffic controllers, TSA, CBP and the entire travel experience as a means to achieve political ends,” it said in a statement.

The measures come as a massive winter storm bears down on the Northeast U.S., which could disrupt airline flights for days.

Airlines have canceled thousands of flights through Monday and waived cancellation and change fees for airports spanning Virginia to Maine ahead of the East Coast blizzard.

— CNBC’s Garrett Downs contributed to this article.

This story is developing. Please check back for updates.