If the battle for Ramadi has already become a brutal slog, pushing the Islamic State in Syria and Iraq (ISIS) from Mosul - Iraq's second city and ISIS' centre of gravity in the country - will be even tougher.
The city, says a senior Western intelligence official, has become a fortress.
A Saddam-era berm that rings it has been repaired by ISIS, the official says, and reinforced with networks of trenches. Belts of villages and towns around Mosul are riddled with mines and improvised explosives, and a network of tunnels has been dug "to rival the Viet Cong".
Even after the persecutions and ethnic cleansing under ISIS, Mosul is still a city of at least 1.3 million. Any operation to clear it could not rely on civilians leaving in advance of military action, as was the case at Fallujah in 2004.
When the United States and Iraq moved to clear Al-Qaeda from Mosul in 2008, it was done with thousands of coalition troops and 10,000 Sunni fighters from the Sahwa, or Awakening, movement.
The campaign to secure the city, which involved defeating a core of about 500 Al-Qaeda fighters, took months. More than 20 heavily fortified command outposts had to be built and manned across the city.
ISIS now has thousands of battle-hardened fighters in its ranks at Mosul. Crucially, it also has a large part of the indigenous population on its side or, at least, ambivalent to it.
"Taking Mosul is going to (require) intensive urban warfare," says counter-terrorism analyst Harleen Gambhir at the Institute for the Study of War.
"(ISIS) has resources to call on. It's not going to be like fighting any other insurgency. And they'll almost certainly use human shields too."
Such a drawn-out and violent conflict would play into the hands of ISIS. Among its faithful, the guiding point of reference is the Battle of al-Ahzab, a 27-day siege of Yathrib in AD627.
The defenders won by sowing discord and allowing their opponents' divisions to split them apart. ISIS propaganda has already declared that "the New Ahzab" is about to begin.