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Al Qaeda bombers on U.S. airliners need prompt attention, but it is Iran, a supporter of terrorism now developing the capacity to fire nuclear-tipped missiles, that may pose the greatest threat to global stability and American security.

That threat can be diminished three ways: by military action, by compromise by Iran's regime, or by a new, less bellicose government taking power in Tehran. The first two appear unlikely, but the third, at least since protests broke out last June after the presidential election, seems more and more realistic. Yet so far the United States and its allies have shrunk from seriously encouraging that third way.

Immediately after the post-election Green Revolution protests began in Iran, some policy makers argued that overt U.S. support would...