Can a preemptive strike ever be justified?
There would probably be a lot more to it than threats. A preemptive strike wouldn’t be decided upon until intelligence had been gathered that showed that the threat against a country was in the final phase of planning. Under those circumstances a country would be faced with either sitting doing nothing and risk being attacked first or act upon the intelligence and strike first. Basically, the alternative to not initialising a preemptive strike (under those circumstances) is risking being the victim of a preemtive strike yourself.
