What the Offensive on Mosul Will Look Like

VIDEO: Iraqi Offensive Battle for Mosul BeginsPlayABCNews.com
WATCH Iraqi Offensive Battle for Mosul Begins

The Iraqi military's looming offensive to retake Mosul from ISIS promises to be not only its largest operation but also its toughest test as ISIS fighters have had more than two years to prepare elaborate defenses inside Iraq's second largest city.

Interested in 2020 Elections?

Add 2020 Elections as an interest to stay up to date on the latest 2020 Elections news, video, and analysis from ABC News.
Add Interest

"The size of Mosul makes this by far the largest task the ISF [Iraqi Security Forces] has undertaken to date, an order of magnitude larger than the liberation battles in cities like Ramadi, Fallujah and Sharqat," Col. John Dorrian, the U.S. military spokesman in Baghdad, told reporters earlier this week.

Last year, American military officials said it could take as many as 20,000 Iraqi military forces to retake Mosul. Since then, the U.S. military's training, advise and assist mission in Iraq has been focused on readying ISF to retake the city held by ISIS since 2014. That training is now almost complete with the last of 12 Iraqi Army combat brigades, numbering between 800 and 1,600 troops, ready to complete its training in a few weeks.

Senior American military officials have said in recent weeks that Iraq's military forces are now prepared to undertake a Mosul offensive but that timing of the operation will be made by Iraqi Prime Minister Haider al-Abadi.

The offensive on Mosul will likely follow the pattern used successfully by the Iraqi military to retake the cities of Ramadi and Fallujah.

In a process that could take weeks, a large number of Iraqi forces will slowly complete the encirclement of Mosul. That will be followed by a push into the center of the city by the elite Iraq special operations force known as the Counter-Terrorism Service, which will be tasked with pushing out the estimated 3,000 to 4,500 ISIS fighters believed to still be in Mosul.

American military advisers working with Iraqi troops will not be engaged in the fighting but will remain at the headquarters of Iraqi military units. Airstrikes for the tough urban fight will be provided by coalition aircraft flying overhead.

"We believe that the Iraqis are well positioned to be successful and of course we'll be there with our strikes so that as the enemy becomes evident, we'll strike them and help the Iraqis advance," said Dorrian.

ISIS fighters have gone to great lengths to prevent that from happening by building berms and trenches along major roadways into the city. They have also placed booby-trapped bombs along roads, bridges and inside buildings in preparation for an urban fight. Giant pits of tire and oil have been readied to create giant dark clouds what would make it difficult for coalition aircraft to conduct airstrikes in the city.

"All these things cause delays and challenges," said Dorrian. "But they're also things that as we train the Iraqis to go into Mosul, a lot of them have received specialized training like explosive ordnance disposal, sniper training, breach training, and there have been warfare-trained so they know how to clear buildings and all these sorts of things."

It is believed some senior ISIS fighters have already left Mosul ahead of the expected Iraqi offensive and a Pentagon spokesman characterized those that have remained as "a demoralized enemy."

"There is a very large scale loss of morale," Capt. Jeff Davis told Pentagon reporters Tuesday.

When the fighting begins, it is believed that as many as 800,000 civilians could flee the city. As part of its planning, the Iraqi government has worked with the United Nations and international relief organizations to build 20 camps to take care of them.

As was done in the battles for Ramadi and Fallujah, the Iraqis will put in place screening procedures to find any ISIS fighters disguised as civilians fleeing the city.

Plans call for local Iraqi police and Sunni tribes to provide security in Mosul once the main fighting has been completed. Then the thousands of Iraqi military forces that fought to retake the city will undergo new training by the coalition in counterinsurgency techniques as ISIS likely morphs away from a combat force into an insurgent role.