The strength of those relationships is likely to determine whether Abadi can cobble together a unity government capable of persuading leading Sunnis to cooperate in the fight against militants from the Islamic State, which has conquered broad swaths of central and northern Iraq. The Obama administration, which hailed Maliki’s decision to step down, has promised to increase its financial and military assistance to Iraq if Abadi’s new government has less of a sectarian bent than Maliki’s hard-line Shiite-dominated one.

Colin Kahl, who formerly served as the Pentagon’s top Mideast policy official, said Abadi will take office with widespread goodwill within the Sunni and Kurdish communities simply because he is not Maliki, who was reviled for instituting policies that discriminated against both groups. But Kahl said Sunni and Kurdish leaders will be looking to Abadi to quickly make substantive moves that show he is genuinely willing to share power. A key early test: whether Abadi puts Sunnis in control of the powerful ministries of defense and interior, which control the country’s military and police forces. Sunnis have wanted those posts for years to ensure that Iraqi security forces aren’t used against them the way they were under Maliki.

“A lot will depend on the initial steps right out of the gate. Who’s his minister of defense? Of interior? How much autonomy will he be willing to give Sunni areas of the country?” Kahl said. “He’ll have a honeymoon period and an opportunity to turn the page, but it’s not inevitable that he’ll take it.”