JUDY WOODRUFF:

Now to the second of our three reports this week on the fight for Iraq.

Militia groups, made up mostly of Shia fighters, and many backed by Iran, have become instrumental in the battle to drive ISIS from Iraq. But their presence on the battlefield makes them a controversial force, one with which the United States has deadly familiarity.

Tonight, again in partnership with the Pulitzer Center on Crisis Reporting, special correspondent Jane Ferguson and producer Jon Gerberg report.