Iran has maintained "red lines" against supporting WMD terrorism but an attack or invasion could change that
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Tehran has also sought at least a degree of deniability in its use of terrorism—a reason it often works through the Lebanese Hizballah to this day when backing terrorists. As Iran expert Kenneth Pollack notes, a chemical or biological attack (to say nothing of a nuclear strike) would lead the victim to respond with full force almost immediately. The use of proxies or cutouts would not shield Iran from retaliation.September 11 has also had a limiting effect. The attacks occurred over a year after the Israeli withdrawal from Lebanon. The tremendous worldwide concern about terrorism, and the active U.S. campaign against Al Qaeda, made Iran's proxies cautious about any attacks that would lead them to be compared to Al Qaeda.Nor do Iran's favored proxies actively seek weapons of mass destruction as does Al Qaeda. They appear to recognize the "red line" drawn by the United States and other powers with regard to terrorist use of these weapons. Moreover, their current tactics and systems enable them to inflict considerable casualties. Indeed, some of the more available types of chemical and biological agents would be difficult for even a skilled terrorist group to use to inflict mass casualties, although the psychological impact would be considerable from even a limited attack with unconventional weapons.Tehran is not likely to change its behavior on this score except in the most extreme circumstances. Traditional terrorist tactics such as assassinations and truck bombs have proven effective for Tehran. Only in the event of a truly grave threat such as an invasion of Iran would many of Tehran's traditional cautions go out the window.
Iran is already known to be a significant sponsor of terrorist groups like Hezbollah. After a U.S. military strike, Iran would be free to greatly increase its material and financial support to terrorist groups, possibly enabling them to strike the U.S. homeland.