Fight to Push ISIS From Its Last Stronghold in Iraq is Imminent, Petraeus Says

— -- The battle to free the Iraqi city of Mosul from ISIS is imminent and will undoubtedly end in victory for the country’s forces, but it will be a fierce fight, U.S. Retired Army Gen. David Petraeus said.

Iraqi forces will soon launch a strategic offensive "orders of magnitude larger than any fight that the Iraqis, with our assistance and coalition support" have taken on in the fight against ISIS in that country, Petraeus told ABC News' Martha Raddatz on "This Week."

He said ISIS will fight hard to maintain its last stronghold in Iraq, which it has controlled for more than two years.

"There will be a ring of fire. They have already lit it supposedly," said Petraeus, who commanded U.S. forces during the Iraq War and drove al-Qaeda out of that country. "There will be dug-in troops, there will tunnels. There will be suicide bombers, improvised explosive devices."

"At the end of the day the Iraqi forces, with our assistance, are going to prevail," he said. "There's no question about the outcome of the fight. The challenge here is actually after the fight, it's governance."

After ISIS is defeated in Mosul, the work will be to re-establish Iraqi governance in the city, he said.

The retired general said forces backed by powerful American airpower and artillery are amassing on the outskirts of Mosul in preparation for the battle. He said U.S. military officials are not releasing a timeline for the attack on the city, but that it is no secret the fight is about to happen.

"The Islamic State fighters in Mosul are dead men walking, and I think they increasingly know it," Petraeus said. "They're even trying to desert and they're being executed."

If the Iraqis are unable to establish a government in the city, a new terror threat could emerge, Petraeus said.

"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.

About the ongoing Syrian civil war and the intensifying battle for the city of Aleppo, Petraeus said fighting there is even more complex than in Iraq. And he seemed to criticize the current U.S. administration for a hollow response that is being taken advantage of by the Syrian regime and its ally Russia.

"It's often said that there is no military solution to the challenges of Syria," he said. "I'm not sure that Putin and [Syrian President] Bashar Al-Assad got that memo because they seem to think they can, indeed, have a military solution and they're trying to do that to 275,000 people trapped in Aleppo right now as an example."

Petraeus continued that "You may buy that there's no military solution, but absolutely if you do not change the military context on that battlefield, there's not going to be any meaningful diplomacy. And that's the situation we're in now."

While critical of the current situation, Petraeus was quick to salute Secretary of State John Kerry's work toward a diplomatic solution to end the violence in Syria.

But he said "the way to change the context is to give additional weapons, to give anti-tank guided missiles and indeed some anti-aircraft missiles to the opposition" fighting the Al-Assad's forces.

The retired military leader also brought up the possibility of a "safe zone" or no-fly zone in Syria and said it is time to consider such possibilities without antagonizing Russia, which is backing the Al-Assad regime.

"I think it is time that we undertake these without being absolutely provocative in what we do to Russia, but be firm," Petraeus said.

As to Russia's larger objectives in the world, Petraeus said Russian President Vladimir Putin "wants to recreate as much of the Soviet Union as he can through a variety of different means. He's invaded parts of Georgia, took Crimea, southeastern Ukraine, bases in other countries."

Pressed on whether the U.S. is in danger of conflict with Russia, Petraeus said, "We have to show a degree of firmness that is going to be unmistakable to [Putin], and by the way, in cyberspace as well."