In October, Iranian Gen. Qassem Soleimani met with his Iraqi Shiite militia allies at a villa on the banks of the Tigris River, looking across at the U.S. Embassy in Baghdad.

The Revolutionary Guards commander instructed his top ally in Iraq, Abu Mahdi al-Muhandis, and other powerful militia leaders to step up attacks on U.S. targets using sophisticated new weapons provided by Iran, said two militia commanders and two security sources briefed on the gathering.

The strategy session, which has not been previously reported, came as mass protests against Iran's growing influence in Iraq were gaining momentum, putting the Islamic Republic in an unwelcome spotlight.