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.
Two Guards' units, dispatched from Iran's western border provinces on Wednesday, were tasked with protecting Baghdad and the holy Shiite cities of Karbala and Najaf, these security sources said.
Iraq's military was trained and equipped by the U.S. and coalition forces. When attacked by insurgents in several cities this week, Iraqi soldiers surrendered without a fight. What went wrong? What is the U.S. prepared to do? WSJ's Jason Bellini has #TheShortAnswer.
The involvement of Iran would pose yet another security challenge for the White House, and raises the prospect of the U.S. and Iran fighting on the same side. The U.S. opposes Iran's support for Syrian President Bashar al-Assad, but with Tehran is jointly supporting Iraqi Prime Minister Nouri al-Maliki.
State Department officials on Thursday refused to outline what steps the Obama administration would take if Iranian forces entered Iraq.
Spokeswoman Jen Psaki said American diplomats who met with Iranian officials in Geneva this week to discuss Tehran's nuclear program didn't raise the issue of the Iraqi crisis.
"We've encouraged them to play a constructive role in Iraq," Ms. Psaki said about the Iranians.
Iraqi Foreign Minister Hoshyar Zebari, reached by phone in London, said of the report that Iran's Revolutionary Guards were entering the fight: "Frankly I have no idea about that. I am in London now."
Syria's conflict has turned Iraq into an important operational base for Iran to aid another ally, the Assad regime, which is dominated by an offshoot of Shiite Islam. Shiite militia trained by Iran, weapons and cash have flowed from Iran to Syria via Iraq.
"Iraq is viewed as a vital priority in Iran's foreign policy in the region and they go to any length to protect this interest," said Roozbeh Miribrahimi, an independent Iran expert based in New York.
