If you looked across the tarmac at the Great Falls, Montana, airport in April, you likely would have been surprised to see a fully marked Russian Air Force jet parked nearby. Its mission that week would have been even more puzzling: The unarmed Tupolev Tu-154M spent four days flying over some of the most sensitive military bases in the US, including the complex in the Nevada desert known as Area 51.

The surveillance flights, all announced and conducted with American personnel onboard to monitor them, were part of a lingering legacy of the Cold War. Authorization under the long-standing treaty known as “Open Skies” made them routine and uncontroversial—at least until Monday night.

That’s when House representative Eliot Engel, the Democrat of New York and the chair of the House Foreign Affairs Committee, sent a letter to White House national security advisor Robert O’Brien saying he was “deeply concerned” by reports that President Donald Trump was considering withdrawing from Open Skies. That would be the latest in the administration’s efforts to unwind many of the multilateral agreements, institutions, and treaties that have helped govern the world and keep peace since World War II.

“[I] strongly urge you against such a reckless action,” Engel wrote. “American withdrawal would only benefit Russia and be harmful to our allies’ and partners’ national security interests. ... The US should prepare for the challenge that Russia presents—not abandon mechanisms that provide the US with an important tool in maintaining surveillance on Russia.”

While the Trump administration and Capitol Hill allies like senator Tom Cotton, the Republican from Arkansas, have long expressed frustration with the deal, Monday’s movement seemed to blindside foreign policy and arms control experts, who quickly expressed puzzlement and outrage that Trump would unwind what’s been seen as a cornerstone of global defense. The former ambassador to Russia, Michael McFaul, tweeted “Please tell me this can’t be true.”

The treaty, which primarily focuses on the US and Russia, actually has a total of 34 signatories across Europe and North America, and allows for countries to conduct structured but almost unimpeded surveillance flights by specially outfitted aircraft to monitor each others’ militaries. Over the last 16 years, the treaty has enabled nearly 200 flights by the US over Russia and more than 70 flights by Russia over the US.

If the Trump administration does pull out, the collapse of the Open Skies agreement would be the latest in a series of little noticed but significant moves by the White House to undo the patchwork of arms control agreements that have kept at bay a new nuclear arms race between the two nuclear superpowers. Earlier this year, Trump withdrew from the 1987 Intermediate-Range Nuclear Forces Treaty, which limited ballistic and cruise missiles with ranges between 500 and 5,500 kilometers, saying that Russia no longer abided by the treaty anyway.

The Open Skies Treaty continues to have unmatched value.

The Russian Open Skies flights over the US often make headlines, as people wonder why Russian surveillance planes are flying overhead, but the US also conducts similar flights over Russia—and in 2018 actually had one such flight over Ukraine to monitor Russia’s military buildup in the territory it seized there in 2014. (America’s fleet of Open Skies aircraft—aging, problem-prone OC-135B planes—is based at Offutt Air Force Base outside Omaha, Nebraska.)

The flights are closely monitored and highly structured; as the State Department’s fact sheet explains, “The treaty limits all optical sensors, including electro-optical, to 30-cm resolution; a level that allows parties to distinguish between a tank and a truck and is of similar resolution to imagery available from commercial sources like Google Earth.”