 -- Iraqi Prime Minister Haider al Abadi has announced the start of the long-awaited offensive to retake Mosul, Iraq's second largest city, which was seized by ISIS in 2014. The battle for Mosul is seen as a pivotal moment towards defeating ISIS in Iraq.

Al Abadi's announcement was made on state television early Monday morning local time in the form of a brief written statement.

"The bell of liberty has been rung and operations to liberate Mosul have started," said Al Abadi, while wearing a military uniform and flanked by senior Iraqi military officers. "Very soon we will be among you to raise the Iraqi flag."

"The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead," Defense Secretary Ash Carter said in a statement. "We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL's hatred and brutality."

"Godspeed to the heroic Iraqi forces, Kurdish Peshmerga, and Ninewa volunteers. We are proud to stand with you in this historic operation," Brett McGurk tweeted following Abadi's announcement. McGurk is the Special Presidential Envoy for the Global Coalition to Counter ISIL.

The Iraqi military is expected to face a tough fight in Mosul, where the 3,000 to 4,500 ISIS fighters in the city have prepared elaborate defenses in anticipation of a offensive.

It is unknown how large a force the Iraqi military will use in its offensive, but American military officials have estimated that it could take as many as 20,000 Iraqi troops to push ISIS out of the city. The Iraqi forces moving on Mosul will include at least 10 Iraqi Army brigades and two Kurdish Peshmerga brigades, all trained by the U.S. led coalition in Iraq.

"This operation to regain control of Iraq's second-largest city will likely continue for weeks, possibly longer," said Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force Operation Inherent Resolve. The general said the offensive would include U.S. air support, artillery, intelligence, advisors and forward air controllers, but that "the thousands of ground combat forces who will liberate Mosul are all Iraqis."

Al Abadi's announcement most likely means that Iraqi and Kurdish forces will begin what could be a long process to encircle Mosul, as occurred in the successful operations in Ramadi and Falujah. After that process is completed, Iraqi and Kurdish forces will begin to push into the center of Mosul, which is expected to be an extended fight.

Townsend called ISIL not only a "threat in Iraq and Syria, but to the region and the entire world," and added that the effort to retake Mosul from the terrorist group "may prove to be a long and tough battle."

American military advisers will assist the Iraqi military forces involved in the offensive at the various headquarters levels in the operation. Coalition aircraft have been conducting airstrikes targeting ISIS in the city, and in recent months the number of airstrikes in and around Mosul have increased in an effort to degrade ISIS' capabilities.

The Mosul operation is seen as a defining moment in the battle against ISIS, which seized large areas of northwestern and western Iraq in 2014. Since then, Iraqi offensives have retaken the major cities of Ramadi and Fallujah in western Anbar.

On Saturday, Iraqi President Masoud Barzani issued a statement announcing that the planning for the offensive on Mosul had been completed and the way had been paved to begin the Mosul operation.

Barzani said the Iraqi and Kurdish governments had agreed to establish "a joint higher political committee whose task would be to supervise the affairs of Mosul after the liberation."

"The Iraqi forces, with our assistance, are going to prevail," retired Gen. David Petraeus said on ABC's "This Week." "There's no question. The Islamic State fighters in Mosul are dead men walking and I think they increasingly know it. They're even trying to desert and they're being executed."

The one-time senior U.S. military commander in Iraq and architect of the 2007 surge in Iraq told ABC's Martha Raddatz that the challenge will come after an Iraqi military victory in military. "The outcome is not in question. The question is what happens afterward."

Petraeus said the biggest challenge will be governance.

"Can a government be established that is representative of and responsive to the most complex human terrain in all of Iraq? That's Nineveh province, for which Mosul is the capital," Petraeus said. "That is a huge challenge. And that will determine the outcome here."

Noting the various ethnic populations in the city, Petraeus said there are "a lot of grievances, scores that may be settled."

"In that kind of complexity, governance is going to have to emerge or else you'll start preparing fields for the planting of the seeds of Islamic State 3.0.," he said.

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