In the hours following the deadly terrorist attacks of September 11, 2001, the United States government took the extraordinary step of sealing U.S. borders to traffic and trade by grounding all aircraft flying into or out of the country and imposing a lock-down on the networks of transportation and commerce that are the lifeblood of our economy and society. Given the uncertainty over what might happen next, these emergency procedures were a necessary and appropriate short-term response to the attacks.