BRUSSELS — When Joseph R. Biden Jr. left office as vice president four years ago, anxiety about nuclear weapons was low, save for North Korea. But after four years of Donald J. Trump, President Biden has returned to a world filled with nuclear dangers.
There is little arms control; modern technologies are unrestrained; and the players are more numerous and rapidly building up nuclear stockpiles. As important, Mr. Trump’s transactional, spasmodic, “America First” policies undermined allies’ confidence in American security guarantees.
Many experts are now warning that Mr. Biden must once again make arms control a priority, even if the notion seems as dated as the wide-lapeled suits of the 1970s and ’80s, when complex treaties about “throw weights” and “multiple-entry vehicles” dominated Cold War diplomacy.
Not to do so, they say, risks the acceleration of a nuclear arms race, with new threats to American allies in the Middle East, Europe and Asia.
But few want to discuss the perils, especially in Europe, where nuclear literacy is largely gone and the danger comes from shorter-range nuclear weapons uncovered by any arms control.
To Nathalie Tocci, director of Italy’s Institute of International Affairs, the lack of a debate is shocking. “We barely discuss nuclear,” she said. “On the risk and threat side, there’s no sufficient understanding of how more dangerous it’s becoming.”
The most immediate fix would be to restore American credibility, experts said, though even that may not be easy. The old assurance that the United States would respond with its own arsenal if allies were attacked was a strong barrier to the spread of nuclear weapons. No more, perhaps.
America’s partners in Europe and Asia feel vulnerable. They want reassurance that America’s security guarantees are valid, realistic and reliable, experts said. If not, some would consider going nuclear themselves, openly or secretly.
Just before the last nuclear arms-control treaty with Russia, New START, expired, Mr. Biden extended for another five years. But it does nothing to diminish the threat from more modern technologies, from tactical or medium-range nuclear missiles, or from other nuclear nations.
All other nuclear arms control treaties, including the Intermediate-Range Nuclear Forces Treaty, have lapsed, and Mr. Trump pulled out of the 2015 Iran nuclear deal, which put tight limits on Tehran’s ability to enrich uranium.
“The combination of these challenges raises the nuclear security of our allies anew, as they ask whether they can continue to rely on the United States as they’ve always done,” said Ivo Daalder, a former U.S. ambassador to NATO and president of the Chicago Council on Global Affairs.
“Some allies wonder about the viability and credibility of the U.S. nuclear and security guarantee,” he said.
Doubts about America’s security guarantees are not new, but they are bigger than ever. Charles de Gaulle, who as French president created his country’s independent nuclear deterrent in the 1960s, questioned American willingness to trade “New York for Paris,” and in 2018 Mr. Trump wondered if the United States should go to war to defend Montenegro, a NATO member state.
Given the new reach and capability of North Korea, with missiles that could hit the United States, Mr. Daalder said, Asian allies are asking: “Will you sacrifice us for you? Will you save Seattle at the price of Seoul?”
With no U.S. nuclear weapons there, reassuring Asia is very difficult.
“The Biden nuclear agenda has not so far received the global attention it warrants, especially regarding Asia and China’s modernization,” said Kevin Rudd, the former prime minister of Australia who is president of the Asia Society.
“There has to be sufficient belief in collective deterrence and the American nuclear umbrella to prevent allies from contemplating their own national nuclear breakouts,” he said.
The problem of reassurance Mr. Biden faces is both military and political, said Mark Fitzpatrick, an associate fellow at the International Institute for Strategic Studies. There is the growing threat from nuclear adversaries and less arms control, he said, and then there are the doubts about American willingness to act.
America’s nuclear guarantee “has suffered the most from Trump’s transactional approach to alliances,” he said. “If I were an American ally I’d have to think twice about how much I could rely on the U.S. guarantee, given that this American isolationism and unilateralism isn’t going away.”
Mr. Daalder put together 16 former officials and analysts to write a report on these issues.
Among its recommendations are suggestions to “rebalance the trans-Atlantic partnership” by encouraging Europe to take more responsibility for its own defense and security.
Europeans should fund “real military capabilities” instead of administration and modernize NATO nuclear assets, the report says. Controversially, it recommends that France and Britain join in extending their nuclear deterrents to cover European allies.
The report also urges Washington to resume serious security cooperation with Japan and South Korea and to create an Asian nuclear planning group, including Australia, to bring allies into American nuclear strategy for the region.
If Europe is also vulnerable, anxiety is especially acute in Asia.
“Both South Korea and Japan are under threat from this growing North Korean nuclear arsenal and missile capability,” said Byung-se Yun, a former South Korean foreign minister. “Both countries feel that the current level of extended deterrence is not sufficient to protect us. Nuclear reassurance has become the first and foremost issue for America in Asia.”
Asians worry that Washington will make a deal with North Korea on intercontinental missiles but not shorter-range ones, which could start to decouple American and South Korean interests.
In opinion polls, a consistent majority of South Koreans support acquiring nuclear weapons, and centrist and conservative political parties have called on Washington to station nuclear weapons in the country.
Japan is also vulnerable but is allergic to debating nuclear strategy after Hiroshima and Nagasaki, said Nobuyasu Abe, a former commissioner of Japan’s Atomic Energy Commission and former U.N. under secretary-general for disarmament.
North Korea does not yet have a second-strike capacity, he said, but “we may be overconfident.”
China, rapidly increasing its military budget and nuclear stockpile, is a different matter. “What’s happening to us is China,” Mr. Abe said. “It’s a big dragon but its tail is too short to be seen by Europeans.”
The risk is Taiwan, he said, and how to defend it or deter China from attacking it.
The American policy of “strategic ambiguity” is outdated, he said, “when Xi Jinping is so explicit on Taiwan.”
The United States is 10,000 kilometers away, Mr. Abe said. “So persuade us that you can deter the Chinese. Are you ready to use nuclear weapons to deter China? Washington has never said yes.”
An aggressive Russia presents similar problems for Europe and especially for Germany, with its own nuclear allergy, said Wolfgang Ischinger, chairman of the Munich Security Conference.
The issue is so sensitive that “for the 16 years of Angela Merkel’s chancellorship we have not had a meaningful discussion about deterrence, what it means, do we need it, why, and the substance of NATO’s policy,” he said.
The loss of public confidence in the United States after the Trump years, especially in Germany, is vivid in opinion polls by the European Council on Foreign Relations.
But French ideas about European “strategic autonomy” are risky, Mr. Ischinger said. “NATO is so important to our security, we must avoid sending any signals that we’re considering alternatives.”
Radoslaw Sikorski, a European legislator and former Polish foreign and defense minister, sees danger in Russia’s new weapons, especially without the intermediate-range forces treaty.
“What’s surprising is that this has provoked no reaction in capitals that have come into range, like Berlin,” he said. “There’s nothing like the discussion and reaction produced by a similar move by the Soviets in the mid-1980s, when millions protested and governments were brought down.”
As Mr. Fitzpatrick notes, “vulnerability doesn’t matter if you believe and trust in U.S. security guarantees, because the U.S. has over the horizon stuff that can hit targets in 30 minutes.”
But that, he and others say, is an increasingly big “if.”