BEIRUT—The threat of Sunni extremists eclipsing the power of its Shiite-dominated Arab ally presents Iran with the biggest security and strategic challenge it has faced since the U.S.-led invasion of Iraq in 2003.
With the Islamic State of Iraq and al-Sham, an offshoot of al Qaeda, rapidly gaining territory, Iran deployed Revolutionary Guards units to Iraq, according to Iranian security officials.
Iran has invested considerable financial, political and military resources over the past decade to ensure Iraq emerged from U.S. war as a strategic partner for the Islamic Republic and a strong Shiite-led state. The so-called Shiite crescent—stretching from Iran to Iraq, Lebanon and Syria—was forged largely as a result of this effort.
Iran has also positioned troops on full alert along its border with Iraq and has given clearance to its air force to bomb ISIS rebel forces if they come within about 60 miles of Iran's border, according to an Iranian army general.
The two IRGC battalions that moved to Iraq on Wednesday were shifted from the Iranian border provinces of Urumieh and Lorestan, the Iranian security officials said.
Revolutionary Guards units that serve in Iran's border provinces are the most experienced fighters in guerrilla warfare because of separatist ethnic uprisings in those regions.