"Nothing"
In the case of terror attacks, there ARE no reliable signs of "imminent danger." There are no troops lining up on borders, no missiles being rolled out, no aircraft warming up on runways. There are no diplomatic warnings, no declarations of war, nothing.
So we're justified in attacking anybody anytime based on...nothing? My God, this is insane! It trivializes war and all its consequences to nothing more than an urge born of fear and paranoia. It's a blank check to be a loose cannon. And when we're all done with the killing and destruction, and it turns out to be unjustified, we can say "oh well, you can never be too careful, ya never know where an attack might come from, better safe than sorry". By this logic, the United States can never be truly secure until all the rest of the world lies in ruins and there's nobody left who could possibly pose a threat.
This is a coward's doctrine. I'm ashamed that it's been advanced in my name and the name of my country.