Putin may react against the US decision to permit Ukraine to fire ATACMs missiles [1] onto targets in Russia in the following way.
Putin may stage a low yield nuclear warning (atmospheric or on sea) explosion over international Black Sea air-sea space to scare the international community. The yield may be less than 100 tons (ie. < 0.1 kiloton) using a very small existing or newly developed warhead. Delivery means might be many and varied, eg. a suitcase nuke [2] (with parachute) dropped out of a helicopter, transport plane, boat or conventional submarine or within an aerial, sea surface or under sea drone or a missile warhead.
There might
be no casualties.
The objective would be to shock the world, including
the US, in preparation for the Trump brokered post January 20, 2025 Ukraine-Russia
Peace Treaty.
Ukraine, the US and broader NATO would be more willing to avoid the threat of a seemingly unhinged Putin ushering in a regional nuclear war by giving in to Putin’s territorial and other demands.
Proposed Historical Precedent
Note that in the runup to the 1945 nuclear explosions against Japan a non-combat demonstration explosion was considered [3].
[1] https://www.aljazeera.com/news/2024/11/19/russia-warns-ukraines-atacms-attacks-mark-new-phase-of-war
[2] https://en.wikipedia.org/wiki/Suitcase_nuclear_device
[3] https://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki#Proposed_demonstration