U.S. President Donald Trump said he has “no problem” with a Russian crude tanker delivering fuel to Cuba, appearing to reverse course over his administration’s oil blockade as the island grapples with a deepening energy crisis.

Speaking to reporters aboard Air Force One on Sunday, Trump said: “If a country wants to send some oil into Cuba right now, I have no problem with that, whether it’s Russia or not.”

His comments come as a Russian-flagged oil tanker, the sanctioned Anatoly Kolodkin, makes its way to Cuba carrying an estimated 730,000 barrels of crude oil.

The tanker is reportedly expected to reach port on Monday and is seen as something of a lifeline to the Caribbean nation, which is facing its biggest test since the collapse of the Soviet Union.

Cuba had been heavily dependent on oil supplies from Venezuela, but it has effectively been cut off since early January when the U.S. launched an extraordinary military operation to depose Venezuelan President Nicolás Maduro.

The Trump administration subsequently threatened to impose tariffs on any country that sent crude to Cuba, prompting the likes of Mexico to halt shipments. The Kremlin has previously shrugged off Trump’s tariff threats, pointing out that Washington and Moscow “don’t have much trade right now.”

Cuba’s President Miguel Díaz-Canel said last week that the island hadn’t received oil shipments in more than three months. The communist-run country, which has said it is holding talks with the U.S., has sought to dramatically increase its solar power generation amid the ongoing fuel shortage.

The island of roughly 10 million people has faced a series of power blackouts in recent weeks and the United Nations has warned that Cuban hospitals have been struggling to maintain emergency and intensive care services.

“Cuba is finished, they have a bad regime and they have very bad and corrupt leadership and whether or not they get a boat of oil it’s not going to matter,” Trump said Sunday.

“I prefer letting it in, whether it’s Russia or anybody else, because the people need heat and cooling and all of the other things that you need,” he added.

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.

Cuba’s communist-run government is facing its biggest test since the collapse of the Soviet Union.

U.S. President Donald Trump has ratcheted up the pressure on the Caribbean island since the Jan. 3, military operation to seize Venezuelan President Nicolás Maduro, a long-time ally of Cuba’s government. Cuba said 32 of its citizens were killed in the attack.

Trump has since effectively cut Cuba off from Venezuelan oil, called its government “an unusual and extraordinary threat” and pledged to impose tariffs on any country that supplies it with oil.

Cuban President Miguel Díaz-Canel condemned the move and said: “Surrender is not an option.” He’s since said the government is prepared to hold talks with Washington, albeit “without pressure or preconditions.”

The country has warned that international airlines would no longer be able refuel in the country due to fuel shortages. Gripped by a worsening economic crisis, Cuba’s government recently adopted rationing measures to protect essential services and ration fuel supplies for key sectors.

The plan reportedly includes restrictions on fuel sales, the closure of some tourist establishments, shortening school days and a shorter working week at state-owned companies to four days, from Monday to Thursday.

“The current situation in Cuba is as serious as it has been since the 1990s, when Cuba suddenly had to survive without the support of the Eastern Bloc,” Par Kumaraswami, professor of Latin American Studies at the U.K.’s University of Nottingham, told CNBC by email.

Trump’s tariff threats have created a deterrent for many nations, Kumaraswami said.

Mexico has sent humanitarian aid but suspended oil shipments, as it sought to preserve its relationship with Havana while avoiding Trump’s tariffs.

Kumaraswami said there was “of course frustration with the difficulties of daily life,” but that “many Cubans are resolved to resist threats to their national sovereignty and a new wave of patriotism has emerged.”

Air Canada subsequently has canceled all flights to Cuba amid the fuel shortage, though the airline said Monday that it would bring some 3,000 customers already in the country home over the coming days.

Tourism is a significant source of revenue for Cuba’s cash-strapped government and the country is a popular winter vacation destination for Canadian tourists.

Unlike in previous crises, Cuba’s regime lacks foreign partners that can step in to help, according to Robert Munks, head of Americas research at risk intelligence company Verisk Maplecroft.

“Raising the stakes, the US has also limited Cuba’s access to hard currency and pressured Nicaragua to end visa-free travel for Cubans,” Munks told CNBC by email.

The government’s pledge to increase the use of limited renewable energy sources is likely “too little, too late,” Munks said. He added that outbreaks of civil unrest were possible, given that the island’s domestic energy production is far short of what it needs to keep the lights on.

“An accelerating collapse of basic services will put the regime under extreme pressure to find a negotiated solution,” Munks said.

He added that “the odds are shortening” on Díaz-Canel being forced from power in the months ahead in a Maduro-style managed transition, but Munks said it was more likely that “the regime will try to muddle through” until the U.S. midterm elections in November.

Cuba’s dwindling oil supplies prompted the United Nations to warn of a possible humanitarian “collapse” last week.

“The Secretary-General is extremely concerned about the humanitarian situation in Cuba, which will worsen, and if not collapse, if its oil needs go unmet,” said UN spokesperson Stéphane Dujarric.

A spokesperson for China’s foreign ministry said Tuesday that Beijing “stands firmly against the inhumane actions that deprive the Cuban people of their right to subsistence and development.” They added that China would, “as always,” seek to provide assistance to Cuba.

Russia, meanwhile, has described Havana’s fuel situation as “truly critical” and said U.S. attempts to further pressure the country were causing numerous problems.

“The Cuban government is not going to submit,” Yaffe said. “The fact is, [the U.S. is] going to keep squeezing and the Cubans are going to keep resisting and there’s going to be a lot of unnecessary suffering.”

She added: “I’m a historian and it’s very vainglorious for historians to try and predict the future but we can look at trends — and I can guarantee you that we were here before in the early 1990s where nobody thought Cuba would pull together and pull through — and they did.”