Explainer-Who are Iraq's Iranian-backed armed groups?

By Michael Georgy DUBAI, July 29 - The U.S. and Saudi Arabia have struck Iran-backed Shi'ite armed groups in Iraq, which the allies said were responsible for drone attacks on Saudi oil facilities. The attacks have put the spotlight on the militias that wield enormous political and military power in Iraq. HOW DID THE MILITIAS BECOME INFLUENTIAL IN IRAQ? The Shi'ite groups sprang up as Tehran steadily entrenched itself in Iraqi politics and security matters after the U.S.-led invasion of Iraq…