Russia's parliament starts voting on Tuesday on withdrawing Moscow's ratification of the Comprehensive Nuclear Test Ban Treaty. Here is a look at some of the questions that raises.
WHAT IS THE CTBT?
The CTBT is a 1996 treaty that bans "any nuclear weapon test explosion or any other nuclear explosion" anywhere in the world.
WHY DOES IT SEEK TO BAN NUCLEAR TESTS?
The treaty's preamble cites the goal of reducing and ultimately eliminating nuclear weapons. It says that the test ban, "by constraining the development and qualitative improvement of nuclear weapons and ending the development of advanced new types of nuclear weapons, constitutes an effective measure of nuclear disarmament and non-proliferation in all its aspects".
WHICH COUNTRIES HAVE SIGNED AND RATIFIED?
A total of 187 states have signed the treaty, and 178 have ratified it by passing corresponding laws in their parliaments.
Of the nine countries that possess nuclear weapons:
Britain, France and Russia have signed and ratified.
The United States, Israel and China have signed but not ratified.
India, Pakistan and North Korea have yet to sign or ratify.
SO WHAT IS THE LEGAL STATUS OF THE TREATY?
The treaty has not legally come into force. It can only do so once it is signed and ratified by 44 named countries - the nine with nuclear weapons and 35 others that possess nuclear power and research reactors.
DOES IT HAVE ANY PRACTICAL EFFECT THEN?
Yes, in practice the treaty has created a de facto norm against nuclear testing. No country has carried out a nuclear test explosion since the 1990s with the exception of North Korea, which conducted the most recent of its six tests in 2017 and is under UN sanctions related to its banned nuclear and ballistic missile programmes.
"Since the conclusion and opening for signature of the CTBT, nuclear testing has become taboo," says the Arms Control Association, a US-based non-profit organisation.
The CTBT is "a remarkably effective treaty, even though it hasn't come into force", said Matthew Harries, director of proliferation and nuclear policy at the RUSI think-tank in London.
The treaty's verification regime includes an International Monitoring System (IMS) - a network of observation facilities around the world that are capable of detecting the sound of a nuclear explosion or the associated seismic activity or radioactive fallout.
WHAT IS RUSSIA CHANGING, AND WHY?
President Vladimir Putin said on Oct. 5 that Russia should withdraw its ratification of the CTBT to "mirror" the treaty status of the United States. Russia's lower house of parliament, the State Duma, has moved quickly to make this a reality and has drafted a bill that deputies are voting on this week.
WHY DOES IT MATTER?
The move is part of a pattern of actions and statements by Russia that have increased nuclear tension with the West since the start of the Ukraine war. It follows Moscow's suspension in February of New START, its last remaining nuclear arms treaty with the United States, which limits the numbers of strategic warheads on both sides. Putin has also this year announced the stationing of Russian tactical nuclear missiles in Belarus, a Russian ally that borders Ukraine.
Putin has issued what the West interpreted as nuclear threats since day one of his invasion of Ukraine, when he said that anyone who got in Russia's way would face "such consequences as you have never encountered in your history". With Russia's conventional forces struggling in the war, he is talking up its nuclear might - including the development of next-generation weapons like the Burevestnik cruise missile and Sarmat intercontinental ballistic missile - in order to deter and intimidate his opponents in the West.
SO WHAT COMES NEXT?
Russia says the CTBT move is not a statement of intent to carry out its first nuclear test since 1990, and that it won't test unless the United States does. But the move would provide it with legal cover to test if it wanted to, and some security analysts see a Russian test now as more likely. Putin could be keeping the testing option in reserve in case of a sharp worsening of Russia's fortunes in Ukraine, when he could use it as a dramatic warning signal for the West to back off. Publicly, the Russian president has stated that he is not ready to say whether a test is needed or not.
