As the consequences of the new administration’s approach to our foreign relations become all too apparent — from Ukraine to incipient trade wars — we must not underestimate the damage to the most fragile aspect of American power: credibility. America’s military and economic influence is immense and well understood by friends and foes.

At issue is the president’s word: Will he stand by America’s commitments, legal and moral, in the exercise of American power? If there is doubt, or a clear “no,” our foes will take advantage of the uncertainty, and our friends and allies will look for other ways to ensure their security, no matter how strong our military or economy, including by considering a logical but dangerous option: building their own nuclear arsenals.

Many of the costs of the great Oval Office meltdown between President Donald Trump and Ukrainian President Volodymyr Zelenskyy are visible now. As the American president parroted Moscow’s rhetoric on Russia’s war on Ukraine and berated the victim of aggression, he greatly weakened Kyiv’s defenses and negotiating position. European leaders, already shaken by remarks from Defense Secretary Pete Hegseth at NATO and Vice President JD Vance’s at the recent Munich Security Conference as well as Trump’s new tariffs, are uniting to support Ukraine and oppose Washington in a trade war. More alarming, European leaders are openly questioning their reliance on an America devoted more to its own economic advantage than to a credible commitment to collective security.

All of this gives new stimulus to the spread of nuclear weapons.

Trump’s near panicky exclamation in the Oval Office that Ukraine’s defense of its territory could cause a nuclear war was extraordinary — and extraordinarily dangerous. True, Russian President Vladimir Putin has publicly threatened to use nuclear weapons in Ukraine, and our own intelligence community has validated this concern. We cannot rest easy.

That said, publicly accusing Zelenskyy of not capitulating to Putin’s nuclear threats only encourages Putin to make them again and perhaps even carry them out. And it discourages our NATO allies, and Europeans in general, from relying on America’s commitment to defend Europe in the case of further Russian aggression.

The American nuclear deterrent, of first the Soviet Union and since Russia, is considered the supreme guarantee of NATO’s defense and has helped prevent war — including nuclear war — for more than half a century. Better there were no nuclear weapons anywhere. But since there are, it’s best for their use to be deterred until they can be verifiably eliminated. Until then, amplifying Putin’s nuclear threats, or appearing to be cowed by them, erodes that deterrence. And with that loss of confidence in America comes inevitable pressures for nuclear proliferation.

This is not a theory but a fact. The strong U.S. response to North Korea’s invasion of South Korea in the 1950s and commitment to provide an American security umbrella over Northeast Asia played a key role in Japan’s 1967 decision to rule out the production, possession or introduction of nuclear weapons. In Europe, the Federal Republic of Germany agreed in 1954 to renounce the production of nuclear weapons and was nested in NATO and the U.S. security guarantee one year later. Throughout Asia and Europe, a number of countries have fallen in line and have become leaders in upholding the 1970 Nuclear Non-Proliferation Treaty preventing the spread of nuclear weapons.

The Trump administration’s bullying, hectoring and hostility toward America’s allies and friends have irrevocably deflated American credibility. The end of an era of nuclear restraint is now dangerously in view. Look no further than the words of incoming German Chancellor Friedrich Merz, who recently declared Washington was indifferent to Europe’s fate, suggesting he would look to France and Britain to form a European nuclear umbrella. French President Emmanuel Macron has now announced he will open such a discussion. Coming soon, a more serious debate in Germany over the merits of a German bomb?

In Europe, Turkey and Poland in particular bear watching: President Recep Tayyip Erdogan has already criticized foreign powers for dictating Turkey’s ability to build its own nuclear weapons, and Prime Minister Donald Tusk called last week for Poland to “reach for opportunities related to nuclear weapons.” Uncertainty in America’s course could fuel both fires.

And what of Japan, South Korea and Taiwan facing nuclear-armed China and North Korea with an uncertain America behind them? What also of Saudi Arabia and the United Arab Emirates with Iran closer than ever to its own nuclear weapon? Are they not under new pressure to join the nuclear club as they see an American president succumb to Russian nuclear threats, cut off aid and intelligence support to a friend fighting for its survival and launching a trade war against countries who fought on the U.S. side in two world wars?

A world in which an erratic, transactional and self-isolated America bereft of credibility leaves its friends and allies to look to develop and deploy their own nuclear weapons for their own safety is not a safer one — for them or for us.

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Steven Andreasen, who served as the National Security Council’s staff director for defense policy and arms control from 1993 to 2001, teaches at the Hubert H. Humphrey School of Public Affairs. Anthony Lake was as a national security adviser in the Bill Clinton administration.

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