Donald Trump’s unexpected announcement that he was suspending joint U.S.-South Korean military exercises as part of negotiations with the North has caused consternation in Seoul as well as at the Pentagon. The greatest alarm, however, will be amongst Washington’s major allies in Asia and Europe, already made nervous by the U.S. president’s rhetoric that Washington spends too much money on defending them.

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It’s not yet certain exactly how much training activity the U.S. military will cancel, but it looks to include several major exercises scheduled for August. Particularly over the last two years, the United States has used such drills to unambiguously threaten Pyongyang over its nuclear program, and the Trump administration clearly hopes that by stopping them, North Korea will keep its missile and nuclear warhead test program on ice.

That is not necessarily in itself a bad deal – and if it reduces the risk of escalation and a potentially devastating conflict, President Moon Jae-in’s South Korean government may well be happy. Suggestions Trump was actively considering withdrawing all U.S. forces from the Korean peninsula, however, will likely be more disturbing for Seoul. And for other U.S. allies who have long come to rely on the presence of U.S. forces and joint exercises to deter their enemies, this development would feel very worrying indeed.

More on North Korea Reuters columnists analyze the latest developments on the Korean peninsula

Nowhere is this more true than in Europe, where officials are now increasingly nervous about Trump’s upcoming visit to the annual NATO summit in July. Trump has made no secret of his belief that European states contribute far too little to their own defense, not to mention the 65,000 U.S. forces currently based on the continent. Last year, he openly hectored them on the topic – and after the bad blood of this month’s G7 summit in Canada, there are widespread worries that things will be even more acrimonious this time.

The United States currently has significant additional forces temporarily deployed to Europe to support major exercises in the Baltic states and elsewhere, designed to send an unmistakable message to Russia that Washington and its NATO allies will fight together if attacked. U.S. and Italian suggestions at the G7 that Moscow should be readmitted to that group – from which it was expelled after its 2014 annexation of Crimea – have already undermined Europe’s hopes of a united front against an increasingly assertive Russian president. Any reduction of U.S. military activities would further undermine NATO’s attempts at building a united front.

So far, the United States has not suggested that it will pull any troops out of Europe, much to the relief of Eastern European states in particular. What does make many NATO states more nervous, however, is an agreement that Poland’s nationalistic government – one of the few to embrace Trump and his worldview – may cut with the U.S. president.

According to reports last month, Poland has offered to pay up to $2 billion to Washington to have a U.S. armored division based on its soil, supplementing a small U.S. force already in the country. That would bolster the U.S. military presence in Eastern Europe – but it is also a major departure from other U.S. military deployment deals. Countries that host considerable U.S. forces – Germany, Japan, the United Kingdom, Italy, Turkey and others – often provide free or subsidized basing and other support to U.S. units there. But they rarely, if ever, pay Washington upfront for their presence.

The United States’ other allies worry that the Poland deal may be the shape of things to come, and that the Trump administration will soon start demanding what is essentially protection money to keep U.S. forces on the ground.

At the NATO summit, defense issues may well become intertwined with the Trump administration’s other major foreign policy preoccupation – renegotiating global trade in the United States’ interests. The U.S. president has always been clear that he believes many nations have given the United States a “bad deal,” including those that rely on Washington heavily for defense.

As for the situation on the Korean peninsula, some military experts have already warned that scaling back exercises will inevitably leave U.S. and South Korean forces less prepared for any conflict, particularly over time. On the other hand, the suspensions do give both Washington and Seoul an easy option to ratchet up diplomatic pressure should Pyongyang resume weapons tests.

Major military exercises have always been used as a way of signaling international resolve and increasing or decreasing diplomatic pressure, and Trump’s move on Korea may just be part of that dynamic. What has the United States’ allies most worried, however, is that it could also be the beginning of a much wider U.S. military withdrawal from the rest of the world, particularly regions bordering powerful potential foes like Russia and China.

Such an isolationist approach might be welcomed by many of Trump’s supporters, as well as those who believe recent U.S. military actions in the Middle East and elsewhere have been costly, expensive mistakes.

U.S. military action has not always been a source of international stability over the last two decades. But U.S. presence has helped keep an often uneasy peace in Europe and Asia for almost 70 years. Whatever happens on the Korean peninsula, the greatest lesson from the Singapore summit may be that the world can no longer take that commitment for granted.