I think the calamity comes when we re-define our objectives. For instance, right now the definition of 'terrorist' is fairly clear. But what if we start to consider drug lords terrorists (we probably already do)? Then what about the small dealers under them? Then what about considering the users they sell to terrorists (for funding them)? Then what about considering that drug user's spouse to be a terrorist, for harboring a terrorist? Then it could be fairly easy to imprison a great number of people. But the real danger is that instead of imprisoning everyone we pick and choose who we want to send away forever. Then when someone speaks out against the government in a way that's frowned upon, the government digs up some dirt which can categorize them as a 'terrorist' and they're locked away in a secret prison or executed.

This might sound absurd but I think it's plausible. And it's exactly this kind of worst-case-scenario preparation and fear which gave us the great documents and principles our country was founded upon.