The leader of Lebanon's Hezbollah militant group pledged that "Iran will not be alone" in the event of war with the United States during a mass rally in Beirut in February marking the 40th anniversary of Iran's Islamic Revolution.
"If America launches war on Iran, it will not be alone in the confrontation, because the fate of our region is tied to the Islamic Republic," Hassan Nasrallah said.
From Lebanon and Syria to Iraq, Yemen, and the Gaza Strip, Tehran has significantly expanded its footprint over the past decade, finding and developing powerful allies in conflict-ravaged countries across the Middle East. Hezbollah is one of the most prominent members of the self-styled "axis of resistance," armed groups with tens of thousands of Shi'ite Muslim fighters beholden to Tehran.
Iran has used such groups in the past to strike its regional foes, and could mobilize them if the latest tensions with the United States lead to an armed conflict — dramatically expanding the battlefield.
The militia, whose Arabic name translates into "Party of God," was established by Iran's Revolutionary Guard during Lebanon's civil war in the 1980s. Today it is among the most effective armed groups in the region, extending Iran's influence to Israel's doorstep.
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In a paper for the Brookings Institute earlier this year, former U.S. Assistant Secretary of State Jeffrey Feltman described the group as revolutionary Iran's "most successful export" and Tehran's "multi-purpose tool."
Hezbollah was formed to combat Israel following its invasion of Lebanon in 1982. It waged an 18-year guerrilla war against Israeli forces, eventually forcing them to withdraw from Lebanon in 2000. Six years later, it battled Israel to a bloody stalemate in the Second Lebanon War.
Today, the group has an arsenal of tens of thousands of rockets and missiles than can reach deep into Israel, as well as thousands of highly disciplined and battle-hardened fighters. Hezbollah has fought alongside government forces in Syria for more than six years, gaining even more battlefield experience and expanding its reach.
At home, the group's power exceeds that of the Lebanese armed forces, and along with its allies has more power than ever in the parliament and government.
Despite the rhetoric, Hezbollah says it is not seeking another war with Israel, and it is not likely to join in any regional confrontation — at least not in the early stages — unless provoked. Hezbollah has lost hundreds of fighters in Syria, exacting a heavy toll on the Shi'ite community from which it draws most of its support.
Yemen's Shi'ite rebels, known as Houthis, swept down from the north and captured the capital, Sanaa, in 2014. A Saudi-led coalition entered the conflict on the side of the government the following year. The war has since killed tens of thousands of people and generated the world's worst humanitarian crisis.
Saudi Arabia views the Houthis as an Iranian proxy, and along with Western nations and UN experts has accused Tehran of providing arms to the rebels, including the long-range missiles they have fired into Saudi Arabia. Iran supports the rebels but denies arming them.
The Houthis have given up little ground since the coalition entered the war, and have targeted the Saudi capital, Riyadh, with long-range missiles. Earlier this week they claimed a drone attack that shut down a major oil pipeline in Saudi Arabia, which responded with airstrikes on Yemen's rebel-held capital that killed civilians.
Iran has trained, financed, and equipped Shi'ite militias in Iraq that battled U.S. forces in the years after the 2003 invasion and remobilized to battle the Islamic State a decade later.
The groups include Asaib Ahl al-Haq, Kataeb Hezbollah and the Badr Organization, all three led by men with close ties to Gen. Qassem Soleimani, the leader of Iran's elite Quds Force and the architect of Tehran's regional strategy.
The militias fall under the umbrella of Iraq's Popular Mobilization Forces, a collection of mostly Shi'ite militias that were incorporated into the country's armed forces in 2016. Together they number more than 140,000 fighters, and while they fall under the authority of Iraq's prime minister, the PMF's top brass are politically aligned with Iran.
U.S. forces and the PMF fought side-by-side against ISIS militants after Iraq's parliament invited the U.S. back into the country in 2014. But now that the war is largely concluded, some militia leaders are calling on U.S. troops to leave again, threatening to expel them by force if necessary. This week, the U.S. ordered all nonessential government staff to leave Iraq, amid unspecified threats in the region said to be linked to Iran.
Hamas fell out with Iran after the 2011 Arab Spring uprisings, losing millions of dollars in monthly assistance. The group today is in a severe financial crisis; its employees and public servants in Gaza have not been paid full salaries in years.
Tehran is said to have continued its military support to Hamas' armed wing, but the group appears to get most of its aid from Qatar, making it less likely that it would rally to Tehran's side in a regional conflict. Islamic Jihad, another Sunni militant group, is seen as much closer to Iran but still not as deeply intertwined as Hezbollah or other groups.
Hamas and Islamic Jihad launched hundreds of rockets from Gaza during a bout of fighting with Israel earlier this month. Israel accused Islamic Jihad of triggering the violence, which was the worst since the 2014 Gaza War. The movement did not deny the Israeli accusations.