Shi‘a groups, taking cues from Iran, try to straddle that divide. So Hizballah now operates as a full-fledged political party in Lebanon, with more than 10 percent of the seats in the country's 128-member parliament. In the country's south, it is also much more: a virtual state-within-a-state responsible for education, health care and social services. Iran's most powerful Shi‘a allies in Iraq-the Supreme Islamic Iraqi Council and the Mahdi Army-likewise have chosen to participate in the country's nascent post-Ba‘athi government. In their choices, these groups have been deeply influenced by the example of Iran's successful Islamic Revolution.

This political involvement, in turn, provides Tehran with significant leverage over those societies-influence that the Iranian leadership can use to eliminate ideological adversaries and shape political outcomes. The extent of this authority can be seen today in Lebanon, where Hizballah, with Iran's blessing, is engaged in a slow-motion coup against the pro-Western government of Prime Minister Fouad Siniora. In Iraq too, the presence of Iranian proxies in the halls of power has allowed the Islamic Republic to exert considerable influence over the country's political direction, much to the detriment of the U.S.-led coalition.

FOR WASHINGTON, these differences should matter a great deal. They suggest that-far from the consolidated terrorist threat envisioned in current national-security doctrine-Sunni and Shi‘a groups have very different objectives and vary in their ability to attain them.

Iran is key to this distinction. Its proxies boast the official backing of a wealthy, nearly nuclear patron. Iran's support is financial; U.S. officials now estimate that Tehran "has a nine-digit line item in its budget for support to terrorist organizations." It is also operational, from a deepening Iranian strategic footprint in the Palestinian territories to the provision of arms and training to thousands of Shi‘a in Iraq. Perhaps most significant, however, is the military bulwark that the Iranian regime can offer to its terrorist affiliates against external aggression. A glimpse of this potential was on display back in 2004 when, as part of the deepening strategic ties between their two countries, Iran formally committed to defending Syria in the event of hostilities-a commitment that the Islamic Republic subsequently extended to Lebanon and Hizballah as well. Given Iran's very public atomic advances, this strategic umbrella could soon be nuclear, providing Iranian affiliates far greater freedom of action than ever before.

But this connection can also be a point of weakness. If the Sunni jihadist front can today be characterized by alignment on general ideological principles among like-minded groups, its Shi‘a counterpart is based on a state-centric model, with nearly all of its groups tethered to Tehran in some way. As such, their threat capabilities, political position and, in some instances, their very survival hinge directly upon Iran's largesse. In many ways, this makes them less durable. It is often said that the loss of Osama bin Laden or any of his top lieutenants would do little to cripple the dispersed and decentralized Sunni Islamist movement. The same survivability does not apply to Shi‘a radicals that operate with a more traditional chain of command, and with greater reliance on their chief political and economic patron. The United States must consequently work to leverage this vulnerability, degrading and denying Tehran the ability to maintain its role as a state sponsor of terrorism in the years ahead through stepped-up interdiction of arms shipments from the Islamic Republic, as well as enhanced efforts to curtail contacts between the Pasdaran and the regime's terrorist proxies.

Most of all, however, the forgoing discussion suggests that, if the United States is truly serious about waging a wider war against terrorism, it must be prepared to confront the world's leading sponsor of it. As we are beginning to learn all too well, the costs-and the consequences-of not doing so are far too high.

Ilan Berman is the vice president for policy at the American Foreign Policy Council, and editor of Taking on Tehran: Strategies for Confronting the Islamic Republic (Rowman & Littlefield, 2007).

¹See Rohan Gunaratna, "The Post-Madrid Face of Al Qaeda", The Washington Quarterly, Vol. 27, No. 3 (2004).

²See, for example, Bruce Riedel, "Al-Qaeda Strikes Back", Foreign Affairs, Vol. 86, No. 3 (2007).