all-out nuclear war
Refers to a full-scale nuclear conflict involving the unrestricted use of nuclear weapons.
If an all-out nuclear war breaks out, humanity will likely perish.
核戦争 is a general term for nuclear war, while 全面核戦争 specifically means an all-out, unrestricted nuclear war.
Compound of 全面 (all-out, full-scale) + 核 (nuclear) + 戦争 (war).