Following the defeat of Saddam Hussein in Iraq, Tehran began organizing and arming Iraqi Shiite militias to fight the U.S. and its allies and to undermine the newly emergent government. These forces went on to play a key role in the fight against Islamic State (IS) and, to a lesser extent, against rebel forces in Syria. In collaboration with other Iranian proxies, they have secured control over both sides of an important border area between the two countries—operating, as always, under the direction of the Islamic Republic. It is likely, according to Lazar Berman and Jonathan Spyer, that the militias will next be used against Israel:
[Israel’s] next war in the north may well be a long slog in which its advantages in firepower and numbers only come into play after the IDF wears down and begins to break apart Hizballah’s rocket array and ground forces. . . . But when Hizballah enjoys an Iranian-controlled corridor through which thousands of combat veterans from the Shiite militias can flow, Hizballah fighters will be in a position to withstand much more. . . . So long as they believe that help is potentially on the way, they will likely keep firing. . . .
Israel’s ground forces—in terms of actual combat soldiers—are smaller than many realize, even with the reserves activated. Israel will need significant ground forces if it intends to capture the territory from which Hizballah rockets are launched at Israel. . . . This overstretched ground force will be under even more stress when thousands of Shiite militiamen are thrown into the fight.
Of course, the Shiite militias have never faced an enemy like the IDF in open warfare. Any attempt to rely on ground maneuver as they did successfully in Iraq [against IS] will turn into target practice for Israeli air and armored forces. They will be fighting a highly advanced, highly capable military that is able to target them rapidly with massive firepower. . . . Still, if the Shiite militias learn from Hizballah, and use pre-prepared defenses and tunnels to ambush advancing IDF ground troops, and remain largely underground or in urban areas, they will make Israel’s already daunting challenge in Lebanon even more difficult.