The United States will pull out of a key nuclear-arms treaty with Russia, President Donald Trump announced Saturday during a post campaign rally gaggle with reporters before boarding Air Force One.

America’s withdrawal from the Intermediate-Range Nuclear Forces Treaty, which the U.S. government has accused Russia of violating with a new nuclear-tipped missile, could signal the start of a dangerous new arms race as the United States and Russia develop and deploy a class of shorter-ranger nukes that the treaty had successfully curtailed for more than 30 years.

President Ronald Reagan and Soviet premier Mikhail Gorbachev signed the INF Treaty in 1987 following six years of tense negotiations between the United States and the Soviet Union.

The treaty compelled both countries to eliminate all ground-launched ballistic and cruise missiles with ranges between 310 and 3,400 miles. For much of the Cold War, the Soviets and Americans each deployed hundreds of these intermediate-range nukes in Europe.

If the Cold War had gone nuclear, these fast-striking nukes—1,500 on the Soviet side and 400 on the American side in 1987—could have dealt the first atomic blows, likely beginning a nuclear exchange that could have ended human civilization.

“The treaty whose text is on this table offers a big chance at last to get onto the road leading away from the threat of catastrophe,” Gorbachev said at the treaty signing.

“We can only hope that this history-making agreement will not be an end in itself but the beginning of a working relationship that will enable us to tackle the other urgent issues before us,” Reagan commented.

But Gorbachev and Reagan didn’t count on Trump, his predecessor Barack Obama and Russian counterpart Vladimir Putin. The first sign that the 1987 agreement was in trouble came in 2011, when the Obama administration warned that new, intermediate-range nuclear-armed cruise missile—under development in Russia since 2008—could violate the terms of treaty.

In May 2013, the U.S. State Department first raised the issue with the Kremlin. Later the same year, the White House formally announced that Russia was in violation of the treaty.

“ Large-scale deployment [of missile-defenses] could deprive Moscow of that ultimate security guarantee” that nukes provide. ” — Nikolai Sokov, a fellow at the Middlebury Institute of International Studies at Monterey

But America’s growing concern did not halt Russia’s work on its new nukes. U.S. moves might even have helped to accelerate Russian developments. In 2015 the Pentagon began installing missile defenses in Romania. The non-nuclear SM-3 missile-interceptors are designed to hit ballistic missiles launched by Iran at the United States, and are not capable of stopping intermediate-range nukes launched from Europe.

But the Russians viewed the SM-3s as a threat. “Large-scale deployment [of missile-defenses] could deprive Moscow of that ultimate security guarantee” that nukes provide, Nikolai Sokov, a fellow at the Middlebury Institute of International Studies at Monterey, told The Daily Beast.

Many Russians also believe the United States plans to secretly add nuclear missiles to the European missile-defense sites—in violation of the INF Treaty and as preparation for an atomic sneak attack. The “secret American nukes” fear is not new. Robert Gates, defense secretary under Obama and George W. Bush, warned about that dangerous misconception back in 2009.

The mutual provocation continued under Trump. The former reality T.V. star-turned-U.S. president urged Japan and South Korea to field their own nukes and, in a phone call with Putin in early 2017, reportedly trash-talked New START, the 2011 treaty that limits the United States and Russia to 1,500 deployed nuclear warheads apiece.

Sometime in 2017 the Russian military finally deployed its new intermediate-range missile, the SSC-8, at a site along Russia’s western frontier.

In January 2017 Trump said that “nuclear weapons should be way down and reduced very substantially.” But his administration wrote plans for a host of new nukes, including smaller “tactical” atomic weapons that the White House might be more willing to use than larger, more powerful “strategic” weapons.

Trump’s Nuclear Posture Review, released in early 2018, codified U.S. rearmament plans, effectively mirroring Russia’s own new atomic deployments. “The new policies only increase the chances of blundering into a nuclear war,” Bruce Blair, a Princeton University nuclear scholar, told The Daily Beast.

Russia bent the INF Treaty but Trump broke it by abandoning the agreement instead of using America’s economic and diplomatic power to force Russia back into compliance.

There was a path to a peaceful resolution of the crisis. “I believe there is a face-saving solution,” tweeted James Acton, a nuclear expert with the Carnegie Endowment. “Russia could modify its offending cruise missile to reduce the range.” In exchange, Acton proposed, the United States could have provided Russia access to its European missile-defense sites, so that Russian inspectors could verify that no nukes were present.

Instead, Trump blew up the Reagan-Gorbachev agreement ... and the decades of modest disarmament it encouraged. “Withdrawing from the INF Treaty is a strange way to show how tough we are,” tweeted Jeffrey Lewis, another nuclear expert at the Middlebury Institute.

With the INF Treaty on the scrap heap, there’s no legal or political framework for limiting the proliferation of a dangerous class of nuclear weapons in the arsenals of the world’s leading atomic powers. In a conversation with The Daily Beast, Tom Collina, policy director at the San Francisco-based anti-nuclear-advocacy group Ploughshares, was blunt about the implications: “I think we are in a new arms race.”