This was chillingly illustrated by my colleague Arwa Damon's intense 28 hours embedded with one special forces group, as it plunged deep into Mosul's eastern neighborhoods. The terrifying experience revealed a force seemingly ill equipped and poorly trained for the task at hand.
ISIS fighters were lying in wait to ambush the unit Arwa was with as its commander ordered the convoy on without an apparent "Plan B" or reserve forces to back them up if they got into trouble.
Conventional wisdom is the radical Islamist terror group has had two years to prepare for this defense. Reality is in Iraq alone their fighters have spent more than a decade honing tactics and techniques for fighting an urban guerrilla conflict against conventional forces -- American first and now Iraqi.
US Marines in Falluja
In 2004, I accompanied a group of US Marines in a massive battle to retake the conservative Sunni city of Falluja from radical Islamists. A 45-minute drive from Baghdad, the city had fallen into the hands of groups that would ultimately become the core of ISIS' leadership.
Within minutes of slicing through the defensive sand berms that ringed Falluja, the lightly-armored Bradley fighting vehicle I was in took some intense fire. We plowed on.
Although I didn't know it at that moment, our mission had already changed. The commander had originally been tasked with dropping us off with a forward unit deep in the city but he had been redirected.
He was the QRF, the Quick Reaction Force, for the unit we were to join, but other Marines had come under fire, and there were casualties, so we had to get them out.
Eventually we got to the front line unit, ran house to house, clearing enemy fighters from top to bottom, before moving on to the next street.
There were very few civilians and the Islamists hadn't yet ramped up their suicide bomber factories that churn out brain-washed killing machines. Neither had they developed their tunnel networks allowing them safe movement across the battlefield. They had yet to hone their nihilistic ideology, expand their territory, stockpile vast amounts of weaponry, or attract an international army of ideologues to their cause.
Years later, under an increasingly savage leadership that would see many Islamists join the ranks of the newly created ISIS, the brutality and violence became extreme. ISIS beheaded prisoners and learned how effective suicide bombers could be. New recruits were first asked if they wanted a suicide mission.
Despite this, the well-trained and well-equipped US Marines did not have it easy in their mission to re-take Falluja from the Islamists. It was laborious and highly dangerous.
As I weigh that experience against what we see in Mosul today -- and short of an unlikely ISIS collapse -- the battle to liberate Iraq's second city is going to be even more costly and slow.
Unlike Arwa's experience with Iraqi special forces, the US Marines had good communications, they had a back-up plan, they had a QRF in reserve to support injured or trapped Marines, and they had vehicles better equipped for this type of warfare.
In Mosul, Iraqi special forces are mostly in lightly-armored Humvees. Each vehicle has only one gunner, while the Marines in the Bradleys had multiple shooters.
Several of the special forces with Arwa were wounded, and almost all their Humvees incapacitated -- many had been poorly repaired from previous skirmishes. The attrition rate of men and materiel will be critical in the battle for Mosul.
The Iraqi army lacks well-trained, battle-hardened forces. The same prestige units that accompanied Arwa at the beginning of this offensive in the east of Mosul will be the same units, weeks from now, who will be at the front lines pushing in to the city center. They will be taking the bridges, securing the airport and then finally taking on ISIS in their deeply-embedded positions in the west of the city.
Therefore, the Iraqis cannot afford losses day after day like those Arwa witnessed early on.
Without doubt some of the Iraqi army units are battle-hardened -- they fought to re-take Falluja from ISIS
earlier this year. Some of the same units also fought for control of nearby Ramadi too. But accumulated losses and the sheer scale of the operation to liberate Mosul -- a city once home to more than 2 million people -- are drawing in less battle-ready men and experienced commanders.
In Falluja, ISIS resistance was fiercest in the first week of the offensive with Iraqi troop causalities high. It took around a month before ISIS collapsed and the city re-taken. Mosul is likely to be a different proposition. ISIS hasn't just had the past few months to prepare for the assault they've been told is coming, they've had control of the city for more than two years. They have prepared the perimeter of the city for an attack, hardening defenses in the places they predict they'll be attacked.
Mosul has greater value than Falluja or any other town or city they've controlled in Iraq. For ISIS, once Mosul is lost their self-declared caliphate will wither and they'll be driven back into neighboring Syria.
But what strikes me most when comparing the Marines' assault on Falluja in 2004 and the operation to re-take Mosul in 2016, is the people -- the civilians. While most people had left Falluja, hundreds of thousands of people in Mosul remain in their homes, trapped by the fighting and used by ISIS as human shields. Snipers fire from houses where families are still living, hiding among them knowing that the Iraqi forces can't fire back with impunity.
And as Arwa witnessed in one tragic incident, civilians are incredibly vulnerable. An unarmed old man was shot and killed by the soldiers after he refused to stop when approaching them.
It was the death of innocent civilians at the hands of US troops in Falluja that helped turn the Iraqi population against the once-welcomed liberators. So the support of the people of Mosul could be in the balance as the Iraq government advances.