The U.S. dollar extended its slide against other major currencies on Monday, the latest sign that investors may be starting to shun what has long been the safest haven in global financial markets.

An index that tracks the dollar against a basket of major trading partners fell for a fifth straight day, even as U.S. stocks and bonds rallied. The dollar has fallen by roughly 8 percent this year, trading near a three-year low.

There has been a particularly steep decline since President Trump announced tariffs on nearly every country’s imports a few weeks ago. The dollar has lost value against the euro, the yen, the pound and a host of other currencies, making imports from those countries more expensive for Americans, even before tariffs are applied.

Investors and many of Mr. Trump’s advisers had expected the dollar to strengthen as tariffs were put in place, given the conventional wisdom that the levies would discourage Americans from purchasing imported goods and in turn reduce the demand for foreign currency. Scott Bessent, the Treasury secretary, argued that the dollar’s appreciation would be significant enough to offset a rise in inflation.

In an interview with Bloomberg on Monday, Mr. Bessent sought to push back on concerns about the recent weakening of the dollar, saying it was still “strong” and the “global reserve currency.”

But the magnitude of the tariffs that Mr. Trump has announced has been more substantial than many expected, unleashing turbulence acute enough to raise questions about whether U.S. assets have lost their luster. On multiple days in recent weeks, when the dollar was selling off, so were U.S. stocks and government bonds, a combination that Krishna Guha, vice chairman at Evercore ISI, described as “rare, ugly and worrying.”

In part, the turmoil reflects the confusion about Mr. Trump’s plans for tariffs. Mixed messages about exemptions and pauses, and which products and countries might be hit with new tariffs, have rattled investors who have long seen dollar-denominated assets like U.S. Treasury bonds as the surest thing in finance.

“Both institutional investors and central banks are having to begin to think about what would happen should the dollar and the Treasury market no longer be the safe haven,” said Joe Brusuelas, chief economist at the consulting firm RSM.

Sharp moves in the value of the dollar can have a destabilizing effect on the global economy, because it serves as a central pillar of the financial system. The dollar is on one side of nearly 90 percent of all foreign-exchange trades, according to the Bank for International Settlements, from Americans abroad using their credit cards to large corporations making billion-dollar takeovers. Essential commodities, like oil, are also typically priced in dollars, regardless of who is buying or selling.

Brad Setser, a senior fellow at the Council on Foreign Relations who previously worked at the Treasury Department, said there were reasons not to read too much into the dollar’s sell-off.

For nearly a decade, U.S. assets have been among the best performers in the world — consider the “Magnificent Seven” tech stocks that propelled the S&P 500 and Nasdaq to a series of record highs.

“A lot of the money coming into the U.S. hasn’t been coming to the U.S. seeking safety. It’s been coming to the U.S. seeking yield and chasing the run-up in U.S. equities,” he said.

“In that context,” he added, “when there’s a general move to reduce risk — because the world certainly seems a lot riskier after Trump’s tariff announcement — some of that money that was betting on U.S. outperformance and the U.S. continuing to offer outsized returns is being unwound.”

Economists now see much higher odds of a recession in the United States because of escalating trade tensions. That may mean the Fed will be compelled at some point to start lowering interest rates to protect the labor market. Lower rates make holding dollar-denominated assets less appealing, which could put more pressure on the currency. While the bar for future cuts appears high given that inflation is poised to rise as growth slows, signs that the economy is hurtling toward a recession could change the central bank’s approach.

If that transpired, Christopher J. Waller, a Fed governor, would support cutting rates “sooner and to a greater extent” than initially expected, he said on Monday. In a speech, he also acknowledged the turbulence caused by Mr. Trump’s tariffs, saying it was an “understatement to say that financial markets did not respond well” to them.

The dollar would be destabilized further if Mr. Trump sought to undermine the independence of the Fed, which sets interest rates free from political influence. Mr. Bessent said on Monday that monetary policy was a “jewel box that’s got to be preserved.”

Mr. Trump will have a chance to pick a new chair of the Fed after Jerome H. Powell’s term expires in May 2026. The administration will begin that process in the fall, Mr. Bessent said.

But even Mr. Setser acknowledged that there might be something more fundamentally worrying to the dollar’s slide than simply a shift in expectations about the economic outlook.

“It is not crazy to think that after a period of exceptional policy volatility in the United States and with real risk of recession, that some foreign investors might wonder whether they should continue to put an ever increasing amount of money into the United States,” he said.

In a career spanning more than 30 years, John Pettigrew has seen big changes in the electricity industry. He started out in 1991, working to introduce natural gas-fired power plants to the grid, gradually replacing polluting coal plants. .

Now, once again, he is managing a tectonic shift to an electrified economy that runs on renewable energy like wind and solar power. But these sources of power generation are far trickier to manage than their coal and gas predecessors.

“Effectively, what we’re doing is reconfiguring the whole network,” said Mr. Pettigrew, chief executive of National Grid, which owns and operates the high-voltage electricity grid in England and Wales.

Mr. Pettigrew was emerging from a tunnel nearly 20 miles long that National Grid has bored deep underground at a cost of about 1 billion pounds (about $1.3 billion). The shaft, which workers ride through on bicycles, will carry new cables to feed the power-hungry offices and residential communities of London.

Mr. Pettigrew and his company are in the spotlight these days. The Labour Party government of Prime Minister Keir Starmer, which came to power in July, is taking a close interest in the electric power system, which it sees as a primary vehicle for delivering political and economic goals.

A more robust, versatile grid will be crucial not only for tackling climate change but for securing Britain’s place on the cutting edge of artificial intelligence, which requires vast amounts of power to run data centers.

The government aims for 95 percent of Britain’s electricity to come from what it calls “clean” sources like wind and nuclear by the end of the decade, up from about 60 percent in 2023. At the same time, demand for electric power is expected to surge.

“We haven’t started to think about how seriously we need to invest in our core infrastructures for the resilience of our economy in a digital world,” Dieter Helm, a professor of economic policy at the University of Oxford, said in a recent podcast.

The price tag for an electricity system that can handle such changes is around £40 billion a year from 2025 to 2030, according to the government. National Grid alone has filed documents with regulators to spend as much as £35 billion over five years.

National Grid was founded in 1990 when the Central Electricity Generating Board, which managed the power network in England and Wales, was broken up in an era of privatization. (The company, which is listed in London, also has a large business managing power networks in the United States.) Mr. Pettigrew has run National Grid for nearly a decade, but he may be facing his greatest challenge, industry experts say.

“I think there’s a big question about how can they build rapidly enough all this new infrastructure at the same time as maintaining the same standards,” said Edgar Goddard, a former National Grid executive and now a director of EPNC Energy, a consulting firm.

An electrified economy will require a highly reliable grid for a host of reasons, including national security, analysts say. At the same time, critics of renewable energy say that relying on sources of power like wind and solar, which are by their nature variable, creates new challenges for the system.

On April 2, a parliamentary hearing on the Heathrow outage became a venue for executives from the airport and power companies politely dodging blame. Electricity executives said that there was sufficient power available. Alice Delahunty, National Grid’s president for transmission and a key aide to Mr. Pettigrew, conceded that the fast-changing demands being made of the power system called for a careful rethinking about it’s resilience.

Britain’s high-voltage network, like those of other countries, used to be relatively simple, bringing electricity from large generating plants — often near where the coal burned in them was mined — to London and other cities.

Now Mr. Pettigrew is extending National Grid’s tentacles toward the coasts, sometimes through scenic areas, to capture new sources of electricity like the giant offshore wind farms now being built in the North Sea.

He also must make sure the system can carry a lot more power.

Demand for electricity, which has been sluggish in recent years, is expected to double in the coming decades as more drivers take the wheel of electric vehicles and data centers spring up to handle everything from financial services to artificial intelligence.

There is already a long line of wind farms, battery storage facilities and data centers waiting to hook up to the grid — sometimes with increasing frustration. “Their connections process is very poor,” James Basden, a founder of a power storage company called Zenobe Energy, said about the large power operators.

A small industry has sprung up to advise companies on how to navigate the gauntlet of securing access to the grid. “We’re seeing huge demand,” said Simon Gallagher, managing director of UK Network Services, one of those firms.

The government is betting that installing swaths of wind turbines — both on land and in the seas off Britain’s coasts — as well as thousands of miles of high-voltage cables will attract investment, nurture clean tech jobs and reduce the country’s vulnerability to price swings in energy like those that occurred after Russia’s 2022 invasion of Ukraine that led to reduced supplies of natural gas.

Since that invasion, high energy costs have been a major issue in Britain and across Europe, where governments have been forced to spend heavily to help households pay their bills.

Some analysts, though, say the huge costs of installing a new energy system may at least partly cancel out the low running costs of wind and solar. “There’s a lot of infrastructure that needs to be built and that’s going to be paid either by taxes or electricity prices,” said Chris Wilkinson, a senior analyst at Rystad Energy, a consulting firm.

Much is at stake for Britain and the wider clean energy industry. If the government’s ambitions prove unrealistic, that could be a blow to the industry, which is already under fire from the Trump administration in the United States.

It certainly won’t be easy to rewire Britain. National Grid is working on 17 large power projects. Some of the schemes involve laying cables for miles offshore to transfer electricity from clusters of wind farms planned for Scottish waters to consumers in England.

Others involve new power lines marching through rural areas on enormous pylons — a prospect that riles up local residents against both the government and National Grid.

The government is taking advantage of its large majority in Parliament to push through legislation curbing the options of opponents of power projects to pursue what it recently called “meritless cases” in court. The government is also planning to offer up to £2500 in compensation over 10 years to people living near the new pylons.

It often takes many years to push projects through the planning system in Britain. Mr. Pettigrew says that process needs to speed up so that Britain can meet its green energy goals.

To achieve anything close to the government’s targets will require an abrupt change in Britain’s leisurely pace of building infrastructure. Offshore wind capacity, for instance, will need to roughly triple. To bring this clean power to consumers will require adding around 3,400 miles of new power lines to the grid, about twice as much as was constructed in the previous decade.

“The way I would describe it is that everybody has to play their part perfectly over the next five years,” Mr. Pettigrew said.