Jim Michaels

USA TODAY

Iraq’s U.S.-backed military is building a ground force and conducting operations in preparation for the largest confrontation yet with the Islamic State — the battle to retake Mosul.

The fight is still months away, but Iraqi ground forces and coalition airstrikes have already begun to isolate militants inside the city, cutting supply lines and trying to weaken militants in advance of a ground assault to take back Iraq's second-largest city.

Iraq’s military has also deployed some of its forces to Makhmur, a base about 60 miles south of Mosul, to “posture for future operations,” said Col. Chris Garver, a U.S. military spokesman.

Iraqi Prime Minister Haider al-Abadi has pledged to retake the city by the end of this year. “We will do what we can to help them accomplish that,” Garver said.

A successful offensive to retake Mosul would strike a major blow to the Islamic State, also known as ISIL or ISIS, which derives much of its power from governing swaths of territory to create a caliphate, or state. The militant group has been under significant pressure in recent months as it has lost a series of towns and cities it had controlled.

Mosul is by far the largest city that remains under the Islamic State's control, having been held by the militant group since it swept into Iraq from Syria nearly two years ago.

“It’s very important to their caliphate,” said Michael Knights, an analyst at the Washington Institute for Near East Policy. “It’s the largest thing they ever controlled.”

For months Iraq’s military and opposition forces in Syria, backed by U.S. airstrikes, have been conducting “shaping operations,” including offensives that have driven militants from Sinjar, in northern Iraq, and Shaddadi, a Syrian town along a supply line between Mosul and Raqqa, the Islamic State’s de facto capital.

The Mosul preparations come after December's successful operation to drive Islamic State militants from Ramadi, an influential Sunni city west of Baghdad.

The ground offensive to take Mosul will be a far more complex challenge than Ramadi. As many as 1 million civilians remain in Mosul, a city about four times the geographic size of Ramadi.

Mosul is defended by thousands of militants, the Pentagon said. Less than a thousand defended Ramadi by the time Iraqi forces pushed in for the final assault.

Militants have had nearly two years to build defenses in Mosul. In Ramadi, they booby-trapped buildings, buried mines and reduced buildings to rubble in an effort to slow attacking Iraqi forces. Iraq’s army can expect similar tactics on a much larger scale.

“The operation in Mosul is much bigger than Ramadi,” said Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff.

U.S. advisers are training and advising Iraqi troops on bases but are not currently accompanying them into battle. The Pentagon has not indicated whether it will recommend that advisers play a more direct role in the Mosul battle.

In Ramadi, the Iraqis relied heavily on their elite counterterrorism forces. In Mosul, they will need a bigger force made up largely of conventional forces, which will need extensive U.S. training to breach minefields and conduct difficult urban fighting.

The Iraqis are planning to build a force of between eight and 12 brigades for the Mosul operation, Army Maj. Gen. Richard Clarke recently told reporters in a briefing from Baghdad. The size of brigades vary, but such a force would consist of more than 20,000 troops, about double what was used in Ramadi.

Generating and training that size force will take time. Iraqi military forces are still tied down in Ramadi, where they are clearing explosives and defending against potential counterattacks. Yanking them from Ramadi risks losing the gains that were made in the December offensive.

The Iraqi force will likely include Kurdish forces called peshmerga plus Sunni and Shiite popular mobilization forces, or militias, which will complicate the political calculus in a country where sectarian tensions run high.

One factor helping Iraq’s military is a momentum shift since the Ramadi operation. The Pentagon has said the Islamic State militants have lost 40% of the territory they had at their peak.

“Momentum is now on our side and not on ISIL's,” Defense Secretary Ashton Carter said recently.

Inside Mosul, there are signs that some fighters have had their pay cut, families of militants are fleeing, and young men are being forced into fighting for the militants.

“Morale isn't necessarily good inside Mosul for ISIS right now,” Garver said.