The U.S. killed at least 25 Ktaib Hezbollah fighters on Sunday night in its first counterstrike in a decade against an Iran-aligned Iraqi Shia militia. U.S. F-15E aircraft struck three sites in Iraq and two in Syria in retaliation for Ktaib’s Friday rocket attack, which killed an American contractor and wounded four U.S. service personnel.

The war against Islamic State in Iraq looks likely to be overshadowed by a growing confrontation between Iranian proxies and the roughly 5,200 U.S. troops in the country.

Ktaib Hezbollah is one of the best-organized and most effective of the Shia militia forces that form Tehran’s main political and military instrument in Iraq. I embedded with the Ktaib in Anbar Province during Iraq’s war with Islamic State in June 2015, spent time with its fighters and commanders, and interviewed its leader, Jamal Jaafar Ibrahimi, who uses the nom de guerre Abu Mahdi al Muhandis, “The Engineer.”

Ktaib, which claims to have 30,000 fighters, is unambiguously an Iranian proxy group, modeled after Lebanon’s Hezbollah. The Islamic Revolutionary Guard Corps created it in 2007 to fight the U.S. presence in Iraq and advance Iranian interests.

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