Define "threat to the world". Can Iran destroy the whole world? Fortunately, not. Can Iran destabilize, hurt and incite violence? Not only can, it is doing it right now.
>>> basing threat levels on previous actions?
That would be stupid because it means that only the second action can be countered. Given how first action can cause devastating consequences, limiting oneself to only reacting to a second one would be terminally stupid. American administration, for example, was justly criticized for sleeping through 9/11, and acting unprepared when it happened. Generals like to prepare for the past wars, it is much easier, but there's no reason to condone such behavior.
>>> who invades countries most often?
Invasions can differ. If Nazi Germany invaded Poland once, and Britain invaded Nazi Germany once, they are not equal.
>>> who's military kills the most people?
Again, depends on the people. If Al-Qaida kills 3000 Americans in 9/11, and then Americans kill 10000 Al-Qaida fighters after that, I'd say good job, they only should have done it earlier, so we would have 3000 less victims.
>>> That saga ended with almost everyone looking bad.
It looks bad to you because you take an impossible position that fighting evil should be done without hurting anyone. It would be nice if there was a nice, fluffy, unicorn-land evil that you could fight by issuing a strongly-worded declaration of condemnation and maybe refuse to send them a Christmas card next time. Unfortunately, real evil is much harder and messier to fight.