The Wall Street Journal reports: 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. [Continue reading…]