Never has the future of nuclear arms control seemed so uncertain.
At risk is not just the collapse of existing treaties, but a whole manner of interaction between Russia and the United States that has been crucial to maintaining stability over decades.
So what’s the immediate problem?
Last week at a meeting of Nato foreign ministers, US Secretary of State Mike Pompeo called Russia out. Moscow, he insisted, had been breaching an important Cold War-era disarmament agreement – the Intermediate Range Nuclear Forces (INF) Treaty.
This 1987 agreement with the ex-Soviet Union removed a whole category of land-based nuclear missiles: those with ranges of between 500 and 5,500km (310-3,100 miles).
Being small, highly mobile, and located relatively close to their potential targets, they were seen as highly destabilising.
In the late 1970s Soviet Russia deployed the SS-20 missile to threaten targets in Western Europe, causing alarm in many Nato capitals.
The US responded by deploying Cruise and Pershing weapons in a number of European countries. But after the agreement, all these weapons were removed and destroyed.
The Trump administration says that a new Russian missile, designated the 9M729 and known to Nato as the SSC-8, breaches the INF Treaty. Mr Pompeo gave Russian President Vladimir Putin 60 days to return to compliance or the US would also cease to honour its terms.
Russia insists that it is abiding by the agreement, and raises concerns about Washington’s adherence to the deal.
So who is right?
The Americans say they have powerful evidence that, over several years, Russia has developed and now fielded a missile that falls within the range that is banned by the INF Treaty.
This by the way is not a new idea raised by the Trump administration. President Barack Obama too was concerned about what the Russians were doing.