In the early days of ISIS's advance there was a fear that they would take Baghdad. While now we know that that was never a possibility for them, lets get thinking. Lets say ISIS pushes on Baghdad and the Iraqi's retreat, continuing southward out of the city. What happens next? Would that prompt an earlier and more direct international intervention? Would the Iraqi state collapse? Would it actually speed up ISIS's demise?
You are talking about a genocial terror army taking a majority Shia city and it also being an armed force at the end of its supply lines.
Frankly if they pushed into the city you would have seen the Sunnis pushed out of Baghdad and the resulting civil war would have been rather apocalyptic potentally with the Gulf States and Iran directly intervening against each other in Iraq.
Millions dead in Iraq and tens of millions displaced if they entered the city. Iraq would be Syria on steroids and it would have caused a global energy shock and a global recession. It came very close to that.
Not gonna happen, the Islamic State only took majority-Sunni areas. They can't take majority Shiite areas without fighting a war to the death against every single shiite there.
It was a defacto partition of Iraq between Sunni Arab, Shiite and Kurdish areas. The only thing that stopped Iraq from shattering is that the Kurds have no interest in being independent while the Turks are strong.
If ISIS can hold the city for a prolonged period then we could see a repeat of the Rape of Nanking .
Additionaly, the Iranians might get directly involved and "re-establish" the Iraqi government more along their lines. How this impacts the Rise of Trump is something I'd rather not think about.
From what I've read there was never any chance that ISIS could take Baghdad. To many Shia, Shia militias, the remnants of the Iraqi Army and not to mention whatever support the rump Iraqi state would receive from the United States/Iran. Basically it wasn't going to happen. MAYBE if say Syria fell, ISIS takes more of it, then with a greater resource base they can compete for the city, but even then it is a stretch.
If everything went so abominably badly that a large ISIS force had been on the verge of taking Baghdad, the level of US airstrikes to prevent that would make the WW2 firebombing of Japan look like a firecracker. The gloves would have been totally off. And if, just IF they kept coming after that, there's a slight chance of the use of tactical nukes ( 10kt and under) on ISIS formations.
The kind of global energy crisis, the bloody civil war in Iraq that would have resulted, the inevitable Iranian invasion to stop ISIS, the economic crash ... all the consequences that would have resulted would have pushed the US to consider crossing that final line.
I don't think the United States is going to nuke anybody unless they are attacked first. I could see boots on the ground and vamping up the bombing, but nuclear weapons. Thats something you use only as a last resort.
Baghdad's Sunni fighters: we are ready for zero hour
In a dilapidated cafe in north Baghdad under a TV set blasting patriotic songs in support of Iraq's embattled prime minister, a young man looked grave.
"Why did the revolutionaries stop?" he asked in a low voice, referring to the Sunni insurgents sweeping across northern Iraq. "Why did they reach Salaheddin [province] and stop? This will be very bad for us, the Sunnis in Baghdad, if they liberate the north and leave us here. We will be under the mercy of the militias. They have to push down, otherwise it will be the end of us."
"There are many men willing to start the fight again but the problem is there is no fear," he said. People were not sufficiently worried about the situation, he said, and did not realise there was no way back. "If we string two Shia on poles for everyone to see, the militias will retaliate and all the men in the area will be forced to carry arms. This is how we start bringing our men together."
A broad-shouldered Sunni commander next to him leaned forward and assured his friend, saying insurgents had set up sleeping cells and were waiting for zero hour to take the war into the heart of the Iraqi capital. "At zero hour, we start our fight by assassinating all the spies and agents. Our neighbourhood, like every Sunni neighbourhood, has many spies and informers. When we assassinate the leaders, the ranks will collapse."
The problem for IS was yes they did their ground work in Mosul and Fallujah of developing cells and networks run by ex-Saddamists who will do anything to get back in power like these guys to take the cities with the help of conventional military might and even more sleeper cells.
You had about a thousand conventional troops attack Mosul from Syria, but around 2,500 men like these who were already in the city and were trained ready and activated as sleeper cells in Mosul for 'Shock and Awe'.
They failed to do their ground work in Baghdad so by the time their conventional and assymetic plan reached the capital the former Saddamists above were unorganized and didn't know what to do. There is no shortage of guys like this thanks to Saddam's radicalizion initiatives to have sent Baghdad to hell just like they almost did mid last decade filling Zarqawi's plan of mass murdering Shia civilians until the Shia were ready to explode.
al-Baghdadi actually originally planned the attack on Mosul to happen later, but when the Iraqi Army broke their Emir in Mosul's computer they had to move up their plans because it was clear who they bought in Mosul and how many cells were loyal as part of their plan.
A later attack on Mosul after they had prepared their cells and organized them in Baghdad and the hundreds of thousands of Sunni men in the city might have joined the fight against the Shia in the city. Then you have a civil war that goes regional and it would be nothing like anything the world has seen since WW2.