What is Russia's 'nuclear doctrine'?

FILE - Troops load an Iskander missile onto a launcher at an undisclosed site in Russia, in this photo released Feb. 2, 2024. Hawks in Russia have called for lowering the threshold for using nuclear weapons. (Russian Defense Ministry Press Service/AP) FILE - A crew prepares a Tu-22M3 bomber as part of drills to train for using tactical nuclear weapons, at an undisclosed location in Russia, in this photo released May 21, 2024. (Russian Defense Ministry Press Service/AP) FILE - A Russian Iskander missile is seen during drills to train the military for using tactical nuclear weapons, at an undisclosed location in Russia, in this photo released May 21, 2024. (Russian Defense Ministry Press Service/AP)

Since Russia invaded Ukraine in 2022, President Vladimir Putin and other Kremlin voices have frequently threatened the West with its nuclear arsenal.