The Iraqi Parliament put in place an important building block of an anti-ISIS strategy on Monday when it approved, in record time, a new power-sharing government led by Prime Minister Haider al-Abadi. Harder work lies ahead as Mr. Abadi, a member of the Shiite majority, tries to manage sectarian rivals in his cabinet and unify a fractured country, all while working with the United States to eliminate a mortal threat from marauding Sunni militants who occupy a huge swath of northern Iraq.

President Obama applied heavy pressure to prod the Iraqis to this point, making clear that a unity government was a condition for American military assistance against ISIS, or the Islamic State in Iraq and Syria. Nuri Kamal al-Maliki, the former prime minister, contributed significantly to the success of ISIS by favoring his Shiite brethren and alienating other communities. Mr. Abadi, who was named to replace Mr. Maliki on Aug. 14, is considered more of a consensus-builder. He has pledged “to work with all communities” and to find the money to help cities and provinces deliver services and to provide housing and education for more than a million Iraqis displaced by ISIS.

But while Americans and Europeans hailed the cabinet as inclusive, there were questions about whether it goes far enough. Two important and controversial positions, minsters of defense and interior, remain unfilled and could inspire more political warfare. Rather than parceling them out on a sectarian basis, Mr. Abadi would be wise to appoint the most competent people he can find at this moment of national peril. His decision to select three former top officials, including Mr. Maliki, for largely ceremonial vice president positions could either be a stroke of political genius or a misplaced gesture than can only compound tensions.

Among Mr. Abadi’s other challenges is to establish a national-guard-like organization designed to integrate Sunni tribes and local militias, like the pesh merga, into a more permanent provincial security structure. He also has to address Sunni grievances, like the release of detainees, and Kurdish complaints over oil exports and territory.

The dangers to Iraq are serious. The resumption of political infighting will only make them worse. Mr. Abadi has his work cut out for him.