Fortunately...
...we live in a country where one is presumed innocent unless proven guilty. Evidently, though, that's not how we currently conduct our foreign policy.
Your suggestion that we engage in pre-emptive strikes because some despot might someday make nuclear weapons is tantamount to writing someone a ticket because they might run a red light sometime in the future.
I can almost buy the arguement that going to war is justified when that despot is proven to actually possess WMD's, but if we're going to spend a hundred billion dollars and risk the lives of our loved ones, I wouldn't mind seeing some real evidence first.