The resignation of Iraqi Prime Minister Nouri Maliki after a bitter final power struggle sets the stage for increasing U.S. arms shipments and military advisors, deepening America's role in a conflict President Obama had sought to avoid.
White House officials, who had urged Maliki to step down, praised him for agreeing Thursday to back Haider Abadi, a less divisive successor who they hope can unite Iraq's political and religious factions against the Islamic State militants who control or threaten much of the country.
"Iraqis took another major step forward in uniting their country," national security advisor Susan Rice said in a statement. "These are encouraging developments that we hope can set Iraq on a new path."
Secretary of State John F. Kerry issued a statement late Thursday commending Maliki for his "important and honorable decision." Kerry urged Abadi and other Iraqi leaders to act quickly to form a new government, "which is essential to pulling the country together and consolidating the efforts of Iraq's many diverse communities against the common threat posed by the Islamic State."
Maliki made his announcement in a nationally televised address from Baghdad, saying he was withdrawing his candidacy in favor of Abadi, who stood next to him. Maliki referred to him as "brother."
"I will not be a reason for the spilling of one drop of blood," Maliki said, according to BBC monitoring. He added: "I say to you, oh people, I do not want any position. My position is your confidence in me, and there is no more sophisticated or honorable position."
Obama now is expected to approve proposals to open the Pentagon spigot wider, sending new shipments of weapons and equipment as well as potentially hundreds of additional U.S. advisors to help Iraqi and Kurdish units with tactics and potentially to call in airstrikes, officials said.
About 1,000 U.S. advisors and other military personnel already are in Iraq, mostly in Baghdad. Obama said Thursday that most of the 20 or so special forces who had helped assess the conditions faced by Yazidi refugees stranded on a mountain in northern Iraq would leave the country in coming days.
"The situation remains dire for Iraqis subjected to [Islamic State fighters'] terror throughout the country, and this includes minorities like Yazidis and Iraqi Christians," Obama said on Martha's Vineyard, where he is on a family vacation. "It also includes Sunnis, Shia and Kurds."
He emphasized America's attempts to provide humanitarian relief, but he also said the Pentagon would continue the airstrikes he authorized last week "to protect our people and facilities in Iraq."
About two dozen airstrikes so far have supported Kurdish forces near Irbil, capital of the semiautonomous Kurdish region, or helped break the militants' siege of Mt. Sinjar, where the refugees had gathered.
But several senior U.S. officials said airstrikes could be launched closer to Baghdad if the extremist fighters show signs of threatening the capital. Asked about that possibility, Rear Adm. John Kirby, the Pentagon spokesman, did not rule it out.
"We stand by, and we're willing to help and to coordinate a little bit with [Iraqi forces], but as the president said, we're not going to become the Iraqi air force," Kirby said. "This is their fight to fight. We're willing to help to the degree we can."
Behind the scenes, the Pentagon and the U.S. Central Command have been preparing for weeks for a rapid expansion of military assistance, which is likely to include Hellfire air-to ground missiles that Iraq can use to carry out its own air attacks, heavy weapons capable of attacking armored vehicles used by the militants, and ammunition.
The U.S. also is likely to deliver additional shipments to Kurdish fighters in the north, who have been battling the militants outside Irbil.
A major U.S. concern is the Mosul dam, the largest in Iraq, which militants seized this month. The extremists could attempt to blow it up, threatening Mosul, about 30 miles downstream.
The U.S. has also sought to enlist allies in helping Iraq. France has begun to deliver weapons to the Kurds, and Germany has signaled it is likely to do so. Saudi Arabia, Kuwait and other Persian Gulf states, who all opposed Maliki, are now expected to increase their support.
On Thursday, U.S. fighters and drones conducted airstrikes northeast of Irbil against two armed vehicles and a U.S.-made armored vehicle, known as a mine-resistant ambush-protected vehicle, or MRAP, the U.S. Central Command said in a statement.
The armed vehicles were destroyed after they were seen firing on Kurdish forces. Forty minutes later, the MRAP was struck, but it wasn't destroyed until it was hit a second time, the statement said.
The White House and its allies were reluctant to support Maliki because they didn't want to side with his authoritarian Shiite-dominated government and security force against the country's minority Sunnis and Kurds.
"We were in a posture of waiting until there was a new government, of saying the onus has to be on them," said a U.S. official, who asked not to be named to discuss the administration's strategy. "But now we are looking to how we can boost them and give them support."
U.S. officials hope Abadi, the prime minister-designate, will be willing to share power with Sunnis, drawing support away from the insurgency. But some officials acknowledge that the U.S. may have to get more deeply involved militarily in coming months to give Abadi time to implement the kind of political measures that Washington has long sought.
Iraq faces a massive challenge regaining the territory it has lost since Islamic State forces swept out of Syria in the spring and raced across the country's west and north, relying on a patchwork of disgruntled Sunni militias to help hold cities and drive out government troops.
But Islamic State may face a challenge keeping more religiously moderate Sunni tribes and former Baathists on its side, said Mitchell Silber, the former head of the intelligence division for the New York Police Department.
"I think they have alliances of convenience," Silber said.
In June the militants took Mosul in three days with a few hundred fighters, as an Iraqi security force of 50,000 that fled their positions, according to U.S. officials. The group now controls oil fields and refineries and could have access to as much as $100 million in annual revenue, the officials said.
The militants' rule in areas controlled by Islamic State has been "brutal but so far effective," said a U.S. intelligence official. He said they have used beheadings and crucifixions to terrorize the population but those tactics may backfire.
Nearly all the Islamic State leaders spent time in American-run jails during the Iraq war, according to U.S. intelligence officials who briefed reporters on condition they not be identified in discussing internal intelligence assessments.
Some of the militants were held only a few weeks after being picked up in a raid, but others spent years in the detention centers, where they were radicalized and made connections with other militants, the officials said. Yet few drew attention at the time, and U.S. intelligence officials know little about their backgrounds.
The organization's leader and self-proclaimed caliph uses a pseudonym, Abu Bakr Baghdadi, and U.S. intelligence agencies don't know his real name, the officials said.