Some observers argue the prospects for nuclear proliferation among U.S. allies appear to have heightened in recent years. Superficially, U.S. President Donald Trump may be responsible. During his 2016 presidential campaign, he made statements suggesting he might tolerate efforts by Japan and South Korea to acquire nuclear weapons. His criticisms of NATO and some of its members have fueled debate in Germany on the desirability and feasibility of the “nuclear option.” Looking more broadly, since the 2008 financial crisis, the U.S. has had to grapple with the management of its security ties abroad amid mounting budgetary deficits and growing public weariness of so-called “forever wars.”

And yet, over two years into the Trump administration, little nuclear proliferation-related activity seems to be taking place among U.S. allies. In fact, some of the most powerful U.S. allies—Japan and Germany—have largely moved away from nuclear energy. What lies behind this current scenario? Perhaps one answer is that the U.S military deployments that underscore U.S. security commitments remain strong.

But how do security guarantees relate to nuclear proliferation? Presidential rhetoric may be important for evaluating the strength of a treaty pledge by the U.S. to defend an ally in the event of attack by a potential adversary like Russia, North Korea or China. However, whether it is more important than other considerations is debatable, since presidential rhetoric may simply be just that: rhetoric. A verbally cited promise to follow through on any pledges recorded in print is simply hard to believe. No country wants to risk the safety of his or her country’s cities for an ally, if they can help it. How can a state, therefore, have any sort of confidence in its alliances?

Thankfully, there is a way. The U.S. deploys a wide range of military forces to shore up local defense and deterrence measures to augment the security of its allies and to advance its own national security interests. U.S. forces stationed in South Korea and Japan, for example, are there for regional contingency if North Korea and/or China were to commit an act of aggression—not to serve as potential targets to trigger wider retaliation by Washington, per the logic of trip-wires. They are also there to kill, making it hard for any potential aggressor to go successfully on the attack without incurring unacceptable costs. So long as the U.S. has such forces on its ally’s territory, or in the theater of operations where a potential war would be fought, the security guarantee is sufficiently strong to have the confidence of its beneficiaries.

It is when the U.S. has unilaterally reduced its forward deployments that its allies become so alarmed they are tempted to acquire nuclear weapons to ensure their own security. A case in point is South Korea in the 1970s.

As it does now, South Korea benefited from a treaty promising that Washington would defend it against external aggression. But when President Richard Nixon came into office in 1969, he spoke of how partners in East Asia and elsewhere would need to bear more of the collective defense burden, especially in light of its failures in Vietnam, rising anti-war sentiment at home and the downturn in the U.S. economy. Yet, the South Korean President at the time—Park Chung-hee—was unfazed by Nixon’s rhetoric. He believed that the United States would reward his country for its contributions in the Vietnam War by retaining its large military presence on the Korean Peninsula.

President Park’s optimism was misplaced. In July 1970, Nixon announced that the U.S. would unilaterally withdraw 20,000 of its military personnel from South Korea within a year, cutting the total U.S. military presence on the peninsula by one-third. The balance of power implications were potentially significant. Since more troop reductions were now conceivable, South Korea would become newly vulnerable to North Korea—an adversary that, at the time, was relatively more industrialized, enjoyed Chinese and Soviet patronage, had a stronger conventional military and had engaged in highly provocative but low-level military activities across the Demilitarized Zone against the South since 1966.

Against this backdrop, President Park put South Korea on the path towards acquiring nuclear weapons. This project failed partly because South Korea needed outside sources for credit and access to reprocessing and enrichment facilities—a vulnerability that the United States used to its advantage to get its ally to curb its activities and to make nonproliferation commitments. Still, South Korea has had persistent interest in fissile materials since the 1970s and continues to undertake continued pyroprocessing activities. Today, South Korea may not be on an aggressive path to acquiring nuclear weapons, but its apparent desire to seek some advantage demonstrates that security guarantees—once broken—can be very difficult to repair, thus prompting countries to reconsider the nuclear option.

The South Korean case illuminates why we are seeing no new efforts to get nuclear weapons, at least by U.S. treaty allies, in the age of Trump. When one looks beyond the rhetoric, actions speak volumes. The Trump administration has reinforced U.S. force posture with additional deployments to Germany. It is seriously considering a permanent military presence in neighboring Poland. Present relations with South Korea have been rocky, but the Trump administration did sign an updated free trade agreement, as well as a temporary agreement, to help cover the costs of 28,500 military personnel stationed there. The U.S. military presence in Japan has remained steady. Indeed, if Trump is serious about containing China, the U.S. military presence will likely increase in East Asia over the long-term.

Dramatic, unilateral troop withdrawals under the Trump administration are still possible, but the historical record indicates that such attempts are generally made early in a presidency rather than later. Nixon, for instance, announced his troop withdrawal out of Vietnam within eighteen months of coming into office.

Putting all rhetoric aside, if the military foundations of U.S. security guarantees remain strong, nuclear proliferation among allies remains unlikely.

Alexander Lanoszka (@ALanoszka, website) is an assistant professor of international relations at the University of Waterloo and an honorary fellow at City, University of London. He recently published the book Atomic Assurance: The Alliance Politics of Nuclear Proliferation with Cornell University Press.

The views expressed in this publication are solely those of the author and do not necessarily reflect the views of the EastWest Institute