nuclear umbrella
Refers to the protection a nuclear-armed state extends to its allies, deterring attacks through the threat of nuclear retaliation. Often used in political and security contexts.
Japan is under the U.S. nuclear umbrella.
The credibility of the nuclear umbrella is being debated.
Compound of 核 (kaku, 'nuclear') + の (possessive particle) + 傘 (kasa, 'umbrella'), a calque of English 'nuclear umbrella'.