My friend Susan Ginsburg, who studied the problem of limiting the capacity of terrorist groups to infiltrate their people into the United States and maintain them here for the 9/11 Commission, has now written a long and densely-argued essay on the problem for the Migration Policy Institute.
The report makes a distinction often ignored in discussions of controlling terrorism, a distinction among three related, but not identical, goals of anti-terror policy: defensive (interfering with specific operations), offensive (identifying and capturing terrorists), and deterrent (discouraging attempts by making them harder and riskier to carry out).
On the problem she calls “terrorist mobility,” Ginsburg argues against another confusion: identifying the narrow goal of keeping terrorists from moving people into place with the broad goal of controlling the flow of people across the borders:
If at the highest level of national strategic and policy discussions, constraining terrorist mobility continues to be casually conflated with maintaining a functional and acceptable immigration system and effective border security—goals that are essential but different and dangerously limiting—we will continue to lose vital opportunities to defend, to deter, and to strike.
You may not want, in fact, to read the whole thing, though you probably want to read the seven-page executive summary. But I devoutly hope that people at DHS, in the White House, and on the Hill are reading the whole thing, and acting on it.