From the Sunni areas around the airport on the West of Baghdad, the Sunnis could pop artillery into heavily Shia Sadr City, just we used to.
Only instead of laser-guided precision munitions, they are more likely to lay down a wide spread of mustard gas and nerve agent, which they recently liberated from al Muthanna.
There is no way the Shia (including Iran) could sit back and leave the holy sites of Karbala (60 miles South of town) to Sunni fundamentalists - it is where the Imam Mahdi, the Shia messiah, is supposed to be. They would have to take at least a few hundred thousand casualties first.
Get your popcorn stockages levelled up now - once the main battle is engaged, you won’t want to have to be making grocery runs.
I agree. The Shia may not be as battle hardened, but they can be as fanatical and no doubt wiil fight hard if properly lead, or even if not. Major use of gas could have interesting international consequences.