The spectre of tactical nuclear weapons use in Ukraine

A vehicle transports a RS-24 Yars strategic nuclear missile along a street during a parade to mark 75 years since the Moscow Victory Parade of 1945 in Moscow, Russia, on Wednesday, June 24, 2020 [Andrey Rudakov/Bloomberg]

As Russia flounders on the battlefields of Ukraine, the once-unthinkable possibility of nuclear weapons use is now on the rise, as President Vladimir Putin’s options for victory narrow. Tactical nuclear weapons have not been part of strategic thinking since the end of the Cold War in 1991. What are these weapons and what would be the significance of their use?

What are they?