The capture of the Iraqi Sunni city of Ramadi by Islamic State (IS) militants has fundamentally changed how war is being waged against the extremists in Iraq's Sunni areas.
As long as government forces had controlled Ramadi, the pro-government Sunni tribal leaders of Anbar Province were opposed to the presence of Iranian-backed Shi'ite militia in the area.
But since May 17, when government forces withdrew from their positions in most of Ramadi, those same Sunni tribal leaders have sought help from Shi'ite fighters in an umbrella group known as Hashid al-Shaabi, or Popular Mobilization Units.
As a result, the Iranian-backed Shi'ite militia fighters have already been deploying into Sunni areas that form the Ramadi front.
Meanwhile, Iranian Defense Minister Hossein Dehghan arrived in Baghdad for talks with senior Iraqi officials on how to expand defense and military cooperation against the IS militants.
It marks an unprecedented level of official military cooperation between Baghdad and Tehran.
Iraqi Prime Minister Haidar al-Abadi, Anbar's pro-government Sunni tribal leaders, and the United States had all been reluctant to deploy Shi'ite militia in Ramadi.
Instead they had advocated the development of locally recruited Sunni forces to combat the Sunni-led IS extremists.
But Anbar's pro-government Sunni leaders now say that the IS militants' capture of Ramadi has shown the government needs the support of the Iranian-backed fighters.
Anbar Provincial Council chief Sabah Karhut told RFE/RL's Radio Free Iraq on May 18 that the provincial council had voted on the issue and had unanimously accepted the need for support from the Shi'ite fighters.
Yusif al-Kilabi, the security spokesman for the Popular Mobilization Units, told RFE/RL on May 18 that thousands of Shi'ite fighters were being sent to the battle, "enough to liberate the city" from IS militants.
Kilabi said he personally attended a meeting with Abadi and Sunni tribal leaders from Anbar Province on May 17 when Abadi gave the order for the Shi'ite deployments.
He said all of the Sunni tribal leaders at that meeting had accepted the need for help from the Shi'ite militia.
Abadi's government says it controls the Shi'ite fighters within the Popular Mobilization Units.
However, with Ramadi under the Sunni extremists' control, the government must now decide whether to continue using Shi'ite militia to defend the roads leading to Baghdad.
Some Iraqis have expressed concerns about whether Shi'ite militia leaders would obey such orders if IS militants launch fresh attacks to the south toward the nearby Shi'ite holy cities of Karbala and Najaf.
Kilabi told RFE/RL that the Popular Mobilization Units were part of the Iraqi security forces and that Shi'ite militia fighters were "ready to go wherever the Iraqi leadership asks us to go."
WATCH: Many Iraqis who have fled the IS advance in Anbar Province have found refuge at a former holiday camp by Lake Habbaniya, just south of Ramadi.
A flood of displaced Sunni civilians pouring into Baghdad from cities like Ramadi and nearby Fallujah also could exacerbate sectarian tensions between Shi'a and Sunni Muslims in the capital.
It is those long-running sectarian tensions that worried Anbar's Sunni leaders about the deployment of Shi'ite militia in predominantly Sunni areas before the weekend.
To be sure, the Popular Mobilization Units played a critical role in the government offensive to recapture Tikrit from IS militants to the north of Baghdad in April.
But some Shi'ite fighters were accused of abuses against Sunni civilians in Tikrit as revenge for mass killings and other atrocities against Shi'a when the IS extremists seized large swaths of territory across northern and western Iraq in 2014.
Kilabi said he didn't expect complaints about Shi'ite fighters in Anbar Province because "it is clear that they need us and they are calling for our help."
Washington's reluctance to bring the Iranian-backed Shi'ite fighters into the battle for Ramadi stemmed from concerns about Tehran's growing military influence within Iraq.
No doubt that issue was raised by the head of the U.S. Central Command, General Lloyd Austin, when he met in Baghdad on May 17 with the Iraqi prime minister to discuss the impact of the fall of Ramadi to the militants.
Abadi and Austin's talks came just hours after Anbar Provincial Council member Athal Fahdawi described the situation in Anbar's provincial capital as "total collapse," prompting the Sunni leaders' call for help from the Shi'ite fighters.
Anbar Provincial Council chief Karhut said on May 18 that members of the Popular Mobilization Units began arriving overnight at a military base near Ramadi.
Hadi al-Ameri, leader of the Badr militia fighters within the Popular Mobilization Units, said on May 18 that Anbar's Sunni leaders should have taken up his offer of military support sooner.
Ameri said he "holds the political representatives of Anbar responsible for the fall of Ramadi because they objected to the participation of Hashid al-Shaabi in the defense of their own people."
But he said his forces were still prepared to join the battle against the IS militants at Ramadi in the days ahead.
A spokesman for Ketaeb Hizballah, another leading Shi'ite paramilitary group within the Popular Mobilization Units, said his organization had fighters ready to join the Ramadi front from three directions.
Spokesman Jaafar al-Husseini said on May 18 that the Kataeb Hizballah reinforcements could start advancing on Ramadi within 24 hours.
Other Shi'ite militia groups announced they also already had units in Anbar Province -- including around the cities of Fallujah and Habbaniyah – that were ready to close in on Ramadi.
U.S. Secretary of State John Kerry said he was confident that the gains made by IS militants in Ramadi would be halted and soon reversed.
"[Islamic State militants'] communications have been reduced, their funding and financial mechanisms have been reduced, and their movements by and large -- in most certainly where there are air patrols and other capacities -- have been reduced but that's not everywhere," he said. "And so it is possible to have the kind of attack we've seen in Ramadi. But I'm absolutely confident in the days ahead, that will be reversed."
The Pentagon vowed a U.S.-led coalition would continue air strikes against IS militants in Ramadi, noting on May 18 that it had carried out 19 air strikes there during the previous 72 hours.