the Soviet Union broke the nuclear weapon test
moratorium that existed between the USA,UK, and USSR
since 1958 with the explosion of the largest H-Bomb ever.

The bomb was named "Ivan","The Monster",
"King of Bombs" or later just "Tsar Bomba".
The name Tsar stands for things build in
Russia that were to big to be used. Like the "Tsar Kolokol"
a bell to big to be carried by any spire or the
"Tsar Pushka" an extraordinarily large howitzer
(never used). "Tsar Bomba" got it