This may be wildly incorrect, but a good offense may be the best defense. Seemingly all other military spending is the nuclear deterrent. An overly basic example: Nuking a country with 90% of money spent on defense means that country's retaliation will be pretty weak. Might be worth nuking them.

Nuking a country where 90% of money is spent on offensive capabilities means the retaliation will be extreme. Probably not worth nuking as you will get attacked back much harder.