We can't.
We can require identification to be shown at every state border we cross, but
-we wouldn't be America any more.
We could require people to check in with their local officials if they want to leave their neighborhood, letting the officials know where they're going and why, but
-we wouldn't be America any more.
We could embed RFID chips in all visitors to our citites or states so they could be tracked, but
-we wouldn't be America any more.
We could record every phone call, email, text messaage or other electronic transmission sent anywhere, to make sure we catch any potential terrorists, but
-we wouldn't be America any more.
We could set up anonymous hotlines where neighbor could report on neighbor, reporting any suspicious activity, but
-we wouldn't be America any more.
We could have government enforcers randomly stop people on the street, in their cars, or in their places of work, and ask for proof the person is a citizen, but
-we wouldn't be America any more.
We could require everyone to prove they never spoke with anyone who might be a terrorist, prove they are in no way related to anyone who might be a terrorist, prove they aren't related to anyone in a country that has terrorists, but not only would we not be America any more, but everyone would be suspect -- and without even going as far as six degrees of separation.
We could ban the sale of any product for which it is believed it might ever be used in any way, in any combination, to terrorize people, and live in a kindergarten-safety-scissor world. I'm not sure if we'd be America any more, but we sure as hell wouldn't like it.
We could allow the President to suspend the right to habeas corpus, allowing us to arrest anyone suspected of or accused by their neighbors of terrorism, citizen or not, without having to bring them to trial,
-as long as the President says it is done in the name of protecting the security of Americans.
We could allow the President alone to decide which electronic communications to monitor without resorting to warrants,
-as long as the President says it is in the name of protecting the security of Americans.
We could allow the President alone to define what consititutes being an enemy combatant, or an unlawful enemy combatant, without stating the definition of these terms,
-as long as the President says it is in the name of protecting the security of Americans.
We could allow the President alone to decide to export persons in American custody to foreign countries where they can be "vigorously questioned,"
-as long as the President says it is in the name of protecting the security of Americans.
We could allow the President alone to decide which actions are "state secrets" and therefore not subject to judicial reivew, or any review at all,
-as long as the President says it is in the name of protecting the security of Americans.
We could allow the President alone to choose whether he will enforce laws passed by Congress, choose how to define "enforce" and choose to change that definition randomly and without explanation,
-as long as the President says it is in the name of protecting the security of Americans.
We could change airline baggage requirements without advance notice, whenever someone reports any method for damaging planes, without checking the validity of the method,
-as long as the President says it is in the name of protecting the security of Americans.
We could do all those things. We have done and are doing some of those things.
As long as people are allowed to move around without checks, as long as people are allowed to carry things with them without checks, we will be unsafe.
How far do we go in the name of safety before we stop being Americans?