Sweden has 65,000 shelters, which would provide space for up to 7 million people, but that leaves an estimated 3 million inhabitants without protection.

At least one European country takes the risk of a nuclear war even more seriously: Switzerland may have fewer people than Sweden, but it has built about four times as many nuclear shelters — easily enough for the country’s entire population and then some.

In Sweden and elsewhere, the nuclear shelters are also supposed to protect the population from other hazards, like a biological weapons attack or more-conventional warfare. Often located in publicly accessible buildings, such as schools or shopping centers, they can usually also be used as storage sites or garages and are funded with taxpayer money..

In contrast, in Switzerland all houses above a certain size must include shelters in the basement, putting the financial burden on citizens themselves. That rule was abolished in 2011 by the Swiss parliament, but reintroduced months later after the Fukushima nuclear accident in Japan.

The accident brought back memories of Chernobyl in 1986, and led to a renewed public debate in Europe over the risk of radiation. In Germany — where public shelters are far less common than in Sweden or Switzerland — Chancellor Angela Merkel decided to abandon nuclear energy entirely, despite having championed it for decades. It was her biggest political U-turn in her now 12 years in office.

Such preparations and protective measures may appear strange to Americans. Public nuclear shelters are practically nonexistent in the United States, although there have been recent reports of an increase in demand.

Until recently, few Swedes knew the location of the closest nuclear shelter in their neighborhood. (The government now offers an online map.) Sweden stopped expanding its shelter network almost two decades ago, when nonproliferation supporters appeared to be on the winning side of history. Then came Iran’s nuclear program, the Fukushima accident, Russian military operations, North Korea’s missile tests — and President Trump.

Whereas confidence among Europeans that President Barack Obama would “do the right thing regarding world affairs” ranged between 70 and 90 percent in a number of surveyed nations during his term, those numbers plummeted after Trump’s inauguration and have only gone down since. Only 7 percent in Spain and 11 percent in Germany now say they have confidence in Trump. Top officials in Germany have also directly contradicted Trump’s North Korea policies, and have voiced concerns that the White House may overreact to nuclear provocations and escalate the war rhetoric being exchanged with North Korea.

Europeans are similarly worried that decades-long nonproliferation efforts could be dismantled virtually overnight, leading to a new arms race. In 2009, the Obama administration negotiated a treaty with Russia in which both countries agreed to cap the number of deployed warheads. Trump reportedly called the agreement a bad deal in his first phone call with Russian President Vladimir Putin earlier this year, although administration officials have since backtracked.

Sweden’s new shelter locations indicate that at least some of the concerns are connected to Russia. One of the regions where most new shelters are expected to be constructed in the coming years is the island of Gotland, where military defenses were recently expanded with the declared aim of stopping a possible Russian invasion.