The Wall Street Journal reports: Iraqi militia fighters are pouring into Syria to reinforce the Assad regime’s siege of rebels in Aleppo, further complicating the tangled web of alliances the U.S. relies on to fight Islamic State, which can turn an ally on one side of the border into an enemy on the other.

The Shiite militias, who have fought alongside U.S.-backed Iraqi government forces against Islamic State in Iraq, are now fighting Syrian Sunni rebels, some of them armed and trained by the U.S.

More than 1,000 Iraqi Shiite militants have traveled from Iraq since early September, joining the ranks of as many as 4,000 others already on the ground near Aleppo, the militia leaders and Syrian rebels said. They make up about half of the regime’s estimated ground force of 10,000.

The siege they are helping to enforce has tilted the battle there in favor of President Bashar al-Assad, whose ruling Alawite sect has drawn on fellow Shiite powers to shore up government forces depleted by deaths, defections and attrition over five years of war: Iran’s Revolutionary Guards Corps, Lebanon’s Hezbollah militia and Afghan Shiite fighters. [Continue reading…]