nuclear defense capability; nuclear deterrent force
Refers to a nation's military capacity to defend against or deter nuclear attack, including both offensive and defensive systems.
That country is strengthening its nuclear defense capability.
核抑止力 specifically means 'nuclear deterrence', while 核防衛力 is broader, encompassing actual defense systems and capabilities.
Compound of 核 (kaku, 'nuclear') + 防衛 (bōei, 'defense') + 力 (ryoku, 'power, capability').