Explainer: What is the Test Ban Treaty and why would a country conduct a nuclear test?

Russia's parliament has passed a law to withdraw Moscow's ratification of the Comprehensive Nuclear Test Ban Treaty (CTBT). The treaty, aimed at reducing and eliminating nuclear weapons, has been signed by 187 states and ratified by 178. The treaty is not legally valid until 44 named countries sign and ratify it. While no country has conducted a nuclear test since the 1990s, Russia's withdrawal raises concerns about a potential resumption of testing, which could prompt other countries to follow suit and lead to a new arms race.