One of these days, the difference between pre-emptive and preventice will be discussed. However, they will not be understood by a fair number of folks...

Pre-emptive is striking before an opponent, when a strike from that opponent is imminent - E.g., the Israeli air-force smashing the Egyptian air-force... on the ground... a day or two before the Egyptians were scheduled to do the same thing.

Preventive would be keeping the Egyptians from building the aircraft in the first place.

Attacking Iran to prevent them from building nuclear weapons would be a preventive strike. I'm certain it would be legal, because George, by the Grace of God, King of America, the Fifth of that name, would say it was.