The Russian military claims it’s making progress on a space plane similar to the U.S. Air Force’s secretive X-37B robotic mini-shuttle.

That in itself isn’t terribly surprising or even, for the United States, particularly worrisome. Lots of governments and even private companies are working on space planes that can launch from rockets or runways, boost into orbit for a period of time then return to Earth for quick refurbishment and re-use.

The tech is pretty basic. But alone among space-plane developers, the Kremlin is proposing to arm its space plane. With nukes.

That’s not only a gross violation of international law, it represents a fairly profound act of hypocrisy on Russia’s part. It wasn’t long ago that the Russian government accused the United States of weaponizing space by sending aloft the nimble, versatile X-37B, basically a quarter-size, remote-controlled version of the Space Shuttle that could, in theory, carry weapons—but does not.

To be clear, a nuclear-armed space plane would be dangerously destabilizing, as it would totally upset the current, tenuous balance of power between the United States and Russia. The Pentagon could respond to a Russian orbital nuke bomber by quickly deploying a space bomber of its own. In other words, an atomic arms race... in space—a development no one should welcome.

Lt. Col. Aleksei Solodovnikov, a rocketry instructor at the Russian Strategic Missile Forces Academy in St. Petersburg who is overseeing the space plane’s development, said the orbital bomber would be flight-ready by 2020. It’s unclear how much money the Kremlin is investing in the project, and how serious senior officers are about actually deploying the space plane, if and when Solodovnikov and his team finish it.

In any event, the military space plane could give Russia a potentially history-altering nuclear first-strike capability.

“The idea is that the bomber will take off from a normal home airfield to patrol Russian airspace,” Solodovnikov said, according to Sputnik, a government-owned news site. “Upon command, it will ascend into outer space, strike a target with nuclear warheads and then return to its home base.”

Thanks to its orbital capability, the bomber would be able to nuke any target on Earth no longer than two hours after taking off, Solodovnikov claimed.

In operational concept, the space-bomber is somewhat different from the X-37B, which launches into orbit atop a rocket like a satellite does and spends a year or more maneuvering around low orbit, reportedly conducting science experiments.

The Russian craft could be closer to Virgin’s family of reusable space planes—the experimental SpaceShipOne and the larger SpaceShipTwo, which is designed to carry paying tourists to the edge of space.

Virgin’s space planes ride piggyback on large “mothership” transport planes until it’s time to launch—at which point they blast off under their own power and head toward the stars.

It’s equally possible that the Russian space-bomber could dispense with the mothership and launch, boost and land while using only its own built-in engines. But the different phases of flight—getting airborne from a runway, then climbing and building up speed and finally making a high-speed lunge into orbit—requires a so-called combined-cycle engine that blends the efficiency of an air-breathing turbofan and the instantaneous power of a rocket.

Combined-cycle engines are hard. The U.S. military’s been working on them for decades, without much to show for it. But one Russian official claimed the Kremlin has already mastered the tech.

“We have accomplished the task of developing a powerplant for a plane that allows it to alternate between the airbreathing regime during a flight in the atmosphere and rocket propulsion regime during a flight in space,” an unnamed official from Strategic Missile Forces Academy told reporters at a military exhibition in October 2015.

Besides the engine, the space-bomber would need a tough, lightweight frame that’s equally adept at traditional runway operations and can survive the stresses of orbital ascents and descents. “We are cooperating with Russia’s Central Aerohydrodynamic Institute on the design of an airframe and the aircraft’s characteristics,” Solodovnikov said.

The new vehicle would be big—weighing up to 25 tons on lift-off, Solodovnikov added. By contrast, the 29-foot-long X-37B weighs fewer than six tons on liftoff. Although to be fair, if the Russian craft indeed launches all under its own power, it would have to be much, much bigger than the X-37B, which gets its initial boost from a disposable rocket.

But the technical challenges are beside the point. While it’s true that Russia—and before it, the Soviet Union—has been tinkering with a military space plane concept since the 1960s without actually producing so much as a working prototype, the hardware isn’t the most vexing aspect of the orbital-bomber program.

The U.S. Air Force, for its part, got pretty close to fielding a space plane called the Dyna Soar back in the early 1960s. There was a version of the Dyna Soar that could have carried one or two atomic bombs and dropped them anywhere on Earth just hours from the word “go.” The Pentagon ultimately cancelled Dyna Soar on cost grounds.

But even if the U.S. military had continued developing Dyna Soar, it would eventually have run into a far more serious obstacle than mere cost—one that still poses arguably the biggest impediment to Russia’s space-bomber.

In 1967, the United States and Russia and 102 other countries signed the Outer Space Treaty, which bans the explicit militarization of space. “States parties to the treaty undertake not to place in orbit around the Earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies or station such weapons in outer space in any other manner,” the treaty reads.

Forty-nine years later, the United States, Russia, and China between them operate hundreds of military satellites. A few have inherently aggressive design features, such as the ability to maneuver close to other spacecraft and potentially disable them by way of extendable claw arms.

But none are solely and strictly offensive weapons. And certainly none pack city-destroying nuclear weapons that can rain down just an hour or so after the command is given. Earth’s surface teems with weaponry, but the world has, so far, managed to keep Earth’s orbit pretty much arms-free.

After the U.S. Air Force launched the X-37B—for scientific purposes, officials claimed—for the first time in April 2010, Russian experts accused the Americans of possibly sneaking a weapon into orbit. The X-37B could “strike global blows on surface targets,” warned Konstantin Sivkov from the Academy for Geopolitical Problems.

In fact, the X-37B is too small to carry a munition for striking Earth. Brian Weeden, a space expert with the Secure World Foundation in Colorado, assessed the X-37B’s potential as a weapon as exactly “zero.” “I don’t see any evidence that it’s being used like that,” Weeden told Space.com.

But the Kremlin’s space-bomber would be a weapon—unambiguously so—and would shatter a half-century of mostly-peaceful space exploration, undoubtedly sparking a terrible diplomatic row and potentially driving the United States and Russia closer to open conflict... on Earth’s surface.