If Iran decides to follow through on its vow of harsh retaliation for the killing of its top general, it can call upon heavily armed allies across the Middle East that are within easy striking distance of U.S. forces and American allies.

It’s a network that was developed over nearly two decades by Gen. Qassem Soleimani, who was killed along with senior Iraqi militants in a U.S. airstrike near Baghdad’s international airport overnight. He enjoyed the fierce loyalty of tens of thousands of fighters in Iraq, Syria, Lebanon, Yemen and the Gaza Strip who received aid, arms and training from Tehran.

Iran has used such groups in the past to strike its regional foes, including Israel, and could mobilize them if the killing of Soleimani ignited an armed conflict — dramatically expanding the battlefield.

Iran’s Supreme Leader Ayatollah Ali Khamenei warned that a “harsh retaliation is waiting” for the U.S. after the airstrike, calling Soleimani the “international face of resistance.”

Read more: AP