You can't defend. You can't prevent. The only thing you can do is detect and respond.
Despite fearful rhetoric to the contrary, terrorism is not a transcendent threat. A terrorist attack cannot possibly destroy our country's way of life; it's only our reaction to that attack that can do that kind of damage.
When people are scared, they need something done that will make them feel safe, even if it doesn't truly make them safer. Politicians naturally want to do something in response to crisis, even if that something doesn't make any sense. But unfortunately for politicians, the security measures that work are largely invisible.
It's frustrating; terrorism is rare and largely ineffectual, yet we regularly magnify the effects of both their successes and failures by terrorizing ourselves.
It is poor civic hygiene to install technologies that could someday facilitate a police state.
There are two types of encryption: one that will prevent your sister from reading your diary and one that will prevent your government.