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…
