nuclear force
Refers to a nation's nuclear military capability, including weapons and delivery systems. Often used in geopolitical and defense contexts.
That country is modernizing its nuclear forces.
The balance of nuclear forces is key to international security.
戦力 means 'military strength' or 'fighting power' in general; 核戦力 specifies nuclear capability.
intermediate-range nuclear forces
Compound of 核 (kaku, 'nuclear') and 戦力 (senryoku, 'military strength').