The United States and Russia have both withdrawn from the Intermediate-Range Nuclear Forces (INF) Treaty.
The end of the treaty, signed by then-US President Ronald Reagan and then-Soviet leader Mikhail Gorbachev more than three decades ago, followed rhetoric by both countries suggesting they would pull out of the agreement.
Earlier this year, Russian parliament approved a bill which would end the country's participation in the INF Treaty.
It came as NATO defence ministers discussed endorsing a list of measures that could be used against Russia if it refuses to comply with a major Cold War-era missile treaty.
President Donald Trump last year announced his decision to withdraw from the INF, which has been described as a cornerstone of global security, accusing Russia and China - which is not a signatory to the treaty - of violating it.
Russian President Vladimir Putin warned the US that if it walked out of the key arms treaty and started developing banned missiles then Moscow would do the same.
What was the INF Treaty?
The INF is a deal signed in 1987 between Russia (then the USSR) and the United States to significantly cut the nuclear weapons capabilities of both sides around Europe.
INF stands for ‘Intermediate-Range Nuclear Forces Treaty’ which states: "Each Party shall eliminate its intermediate-range and shorter-range missiles, not have such systems thereafter, and carry out the other obligations set forth in this Treaty."
What weapons did it ban?
In simple terms, it stopped Moscow from having ground-based missiles that could hit Europe and stopped Washington basing missiles in Europe that could hit Russia.
Neither side was allowed any surface-launched missile that had a range of between 500 and 5500 km (310–3,420 miles).
The US think tank the Arms Control Association says more than 2,600 missiles were destroyed as a result of thE treaty.
But what did it mean for the UK?
It significantly reduced the risk of the UK being hit by a nuclear strike at short notice. As a result, in the early 1990s, the 'four-minute-warning' public alert system ended operations.
The deal also meant an end to the American nuclear-armed cruise missiles that had been controversially based at RAF Greenham Common and RAF Molesworth.
This treaty had no effect on the UK’s own nuclear weapons capabilities or rights, as it was only between Russia and the US.
Why did US President Donald Trump want to pull out of the INF treaty?
The worry in much of Europe is a return to the days of the Cold War when Russia had thousands of missiles that could hit places like London, Paris and Berlin in a matter of minutes.
NATO has previously said the INF Treaty "has been crucial to Euro-Atlantic security and we remain fully committed to the preservation of this landmark arms control treaty".
But the head of the alliance, Secretary General Jens Stoltenberg, has also said it is Russia that jeopardised the treaty by developing new missiles.
'We don't want a nuclear arms race', said the NATO Secretary General in December.
What has Russia's stance been?
Russia denies having breaching the INF Treaty and said the 9M-729 missile fully complied with the agreement.
However, it counter-accused the US of breaching the treaty by placing a missile defence shield (effectively missiles designed to shoot down other missiles) in Europe, and with weapons carried on drones.
Russia's Deputy Foreign Minister Sergei Ryabkov in the past has warned withdrawing from the treaty would be a "dangerous step" that could lead to a new arms race.
He told the RIA Novosti news agency that if the US did pull out of the INF treaty "we will have no choice but to undertake retaliatory measures, including involving military technology".
Then-President Ronald Reagan and General Secretary Mikhail Gorbachev speak after signing the INF treaty in 1987 (Pictures: Ronald Reagan Presidential Library).