Iraq is poised to deploy Shi’ite paramilitaries backed by Iran to Sunni tribal areas west of Baghdad, a move supporters say is needed to defeat Islamic State militants but opponents say could inflame more sectarian violence.

Shi’ite paramilitaries have already played a central, if controversial, role alongside regular army units in recent months in the Iraqi government’s first major successful campaign against Islamic State fighters, helping to capture Saddam Hussein’s home town Tikrit on the Tigris River north of Baghdad.

So far, however, the government has avoided deploying the militia in the Euphrates River valley province of Anbar west of the capital, a vast Sunni tribal homeland that strides the main routes to Jordan and Syria. Baghdad considers Anbar the next target in its campaign to retake territory from the militants. […]