As 2019 rounded to a close, the United States that was up until just a few months ago prohibited by international treaty. The missile blasted off from Vandenberg Air Force Base and splashed down in a patch of the Pacific Ocean, signifying the end of an important arms control treaty signed with the Soviet Union in 1987. The end of the treaty, while not unexpected, represents a worsening of relations between the nuclear powers.

The test took place earlier this month at Vandenberg Air Force Base on the central California coast. The missile flew west for approximately 310 miles, a distance formerly banned by the Intermediate Nuclear Forces Treaty—or INF Treaty for short. The missile was tested just three and a half months after the United States had formally withdrawn from the treaty after repeated allegations of Russian cheating. Here’s the Department of Defense video of the test.

The story goes back to 1987, when President Ronald Reagan and Soviet Premier Mikhail Gorbachev signed the . The treaty between the U.S. and U.S.S.R. was designed to eliminate an entire class of missiles: so-called medium and intermediate-range missiles with ranges between 500 kilometers (310 miles) and 5,500 kilometers (3,417 miles). The net result of this was the elimination of thousands of nuclear-tipped missiles in Europe ready to strike America’s NATO allies, the countries of the Warsaw Pact, and the western Soviet Union.

There were some caveats. The treaty didn’t ban short-range ballistic missiles with ranges of 309 miles or less, nor did it ban intercontinental ballistic missiles with ranges of 3,418 miles or more. This left battlefield nuclear weapons and city-destroying long range nuclear missiles A-OK, though for many the intention was to eventually get to those also.

The end of the Cold War and the dissolution of the Soviet Union pushed nuclear weapons to the back burner, and Russia quietly agreed to inherit responsibility for the treaty.

But the rise of Vladimir Putin marked a downward turn of U.S.-Russian relations, and it’s no great surprise that in 2014 the State Department claimed that Russia had violated the INF Treaty with a new ground-launched cruise missile. Relations with Russia took a further dive under President Trump, and in 2018 the State Department finally put a name to the treaty-offending missile: the . According to Foggy Bottom, 9M729 was allegedly an existing short range cruise missile modified to fly treaty-busting. Russia tried to convince the U.S. and NATO that the missile was not a violation, but the Trump Administration announced in February 2019 it would leave the treaty in six months. By August, the treaty was officially dead.

The U.S. tested this new missile just three and a half months after the end of the treaty, and clearly the effort to build it had been going on for longer than that. The prime contractor for the test was Northrop Grumman, and according to Harvard astronomer and space launch watcher Jonathan McDowell, the missile appeared very similar to NG’s Castor 4 rocket.

Neither country apparently plans to put nukes on its new missiles any time soon, but both Washington and Moscow are back on the long and winding road to doing so. Nuclear-tipped intermediate range weapons are an eventuality, especially for a continent-straddling power like Russia. Most of Russia’s enemies and potential enemies are fairly close by. If Russia deploys them, the U.S. might too and we could be back where we started.

The INF Treaty’s demise is also a poor example to set at a time when the world is trying to restrict the nuclear ambitions of countries like North Korea or Iran. If the major powers can’t abide by arms control treaties, why should anyone else?

One could make a pretty good case that Russia—a country that , , and , is the bad actor here. One could also make the case that Russia needs to be punished for violating the treaty, and the U.S. is simply playing the hand it’s been dealt.

But either way, this is a major moment in a long unraveling between the two countries, each of which has 1,500 nuclear weapons on alert at all times and ready for launch at a moment’s notice.