In the early years of the Cold War, the United States adopted a policy of denying entry visas to foreign nationals on the basis of their political beliefs, a policy now known as ideological exclusion. As the nation recovered from the self-inflicted wounds associated with McCarthyism and the "Red Scare," the policy of ideological exclusion was abandoned.
Now, however, ideological exclusion is back. Indeed, to borrow a phrase, the United States seems to have transformed the war of ideas into a war against ideas. Using authority granted by the USA PATRIOT Act, the Department of Homeland Security has been denying entry to scholars whose views don't match those espoused by the Bush Administration. Tariq Ramadan, a distinguished Swiss scholar who was offered a tenured appointment at Notre Dame in 2004, has been barred from the U.S. Adam Habib, a South African scholar and critic of the war in Iraq, was denied entry when he got off the plane at JFK International Airport in October 2006 en route to meetings at Columbia University and the Social Science Research Council. Ramadan and Habib are not the only foreign nationals who have been affected by the policy of ideological exclusion, but they have become particularly prominent as a consequence of lawsuits filed by professional organizations (the American Academy of Religion on behalf of Ramadan and the American Sociological Association on behalf of Habib) against the Department of Homeland Security.
For more on ideological exclusion, see the information provided by the American Civil Liberties Union here. For information on some of those who have been kept from teaching or addressing professional audiences in the United States since 9/11, see this article in Academe, a publication of the American Association of University Professors.