The battle for Mosul, involving 100,000 Iraqi troops, members of the Kurdish security forces and Shi'ite militiamen, is the biggest ground operation in Iraq since the U.S.-led invasion of 2003. The upcoming phase appears likely to give American troops their biggest combat role since they fulfilled President Barack Obama's pledge to withdraw from Iraq in 2011.
Elite Iraqi soldiers have retaken a quarter of Mosul, the jihadists' last major stronghold in Iraq, but their advance has been slow and punishing. They entered a planned "operational refit" this month, the first significant pause of the campaign.
A heavily armoured unit of several thousand federal police was redeployed from the southern outskirts two weeks ago to reinforce the eastern front after army units advised by the Americans suffered heavy losses in an Islamic State counter-attack.
U.S. advisers, part of an international coalition that has conducted thousands of air strikes and trained tens of thousands of Iraqi ground troops, will work directly with those forces and an elite Interior Ministry strike force.
"Right now we're staging really for the next phase of the attack as we start the penetration into the interior of east Mosul," Lieutenant Colonel Stuart James, commander of a combat arms battalion assisting Iraqi security forces on the southeastern front, said in a Reuters interview late on Sunday.
"So right now, positioning forces and positioning men and equipment into the interior of east Mosul... it's going to happen in the next several days."
That will put U.S. troops inside of Mosul proper and at greater risk, though James said the danger level was still characterized as "moderate". Three U.S. servicemen have been killed in northern Iraq in the past 15 months.
James, speaking from an austere outpost east of Mosul where several hundred U.S. troops are stationed, said the pace of the upcoming phase on the eastern side would depend on resistance from Islamic State, also known as ISIS, ISIL or Daesh.
"If we achieve great success on the first day and we gain momentum, then it may go very quickly. If Daesh fights very hard the first day and we run into a roadblock and we have to go back and go OK that was not the correct point of penetration, it may take longer," he said.
Further integration with the Iraqi troops - to what commanders described as an unprecedented level for conventional U.S. forces - will help synchronize surveillance, air support and force movement, according to James.
"It increases our situational understanding. The man on the ground knows what's going on best," he said. "It's just better when they're on the ground talking to each other and saying, 'Hey, have you looked at that area over there? That's decisive terrain. Have you thought about putting forces there?'"
Mosul, the largest city held by Islamic State anywhere across its once vast territorial holdings in Iraq and neighboring Syria, has been held by the group since its fighters drove the U.S.-trained army out in June 2014.
Its fall would probably end Islamic State's ambition to rule over millions of people in a self-styled caliphate, but the fighters could still mount a traditional insurgency in Iraq, and plot or inspire attacks on the West.
A multi-ethnic city where up to 1.5 million people of a pre-war population of around 2 million are still thought to be living, Mosul is divided roughly in half by the Tigris River. The western section, which Iraqi forces have yet to penetrate, has built-up markets and ancient narrow alleyways which will complicate future advances.
Iraqi Prime Minister Haider al-Abadi had said he would win Mosul back by the end of this year, a deadline now certain to be missed. His commanders say their advance was held up by the need to protect civilians, fewer of whom fled than initially expected.
Inclement weather has repeatedly delayed ground advances which rely heavily on aerial surveillance and air strikes.
Aboard a U.S. Eye in the Sky, Staring Down ISIS in Iraq and Syria
ABOARD A JOINT STARS SURVEILLANCE PLANE, Over Northern Iraq — Flying at 30,000 feet, the powerful radar aboard this Air Force jet peered deep into Syrian territory, hunting for targets on the ground to strike in the looming offensive to seize Raqqa, the Islamic State’s capital.
It was on a mission like this several weeks ago that analysts discovered a hiding place in the central Syrian desert where the Islamic State was stashing scores of oil tanker trucks that provide the terrorist group with a crucial financial lifeline. Acting on that tip and other intelligence, two dozen American warplanes destroyed 188 of the trucks in the biggest airstrike of the year, eliminating an estimated $2 million in oil revenue for the Islamic State, also known as ISIS or ISIL.
Even as the American-led air campaign conducts bombing missions to support Iraqi troops fighting the Islamic State in Mosul, American commanders said the air war would probably play an even greater role in Syria over the coming weeks in the battle to retake Raqqa.
Newly recruited Syrian Arab militia fighters, allied with experienced Kurdish fighters, are encircling Raqqa. But they need allied bombing to weaken and dislodge enemy forces dug in there, and to cut off the ability for the Islamic State to rearm, refuel and reinforce its fighters.
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But with few spies in the city, American officials say assessing the enemy is difficult.
“We’ve spent a lot of time trying to understand the situation on the ground in Raqqa,” Lt. Gen. Jeffrey L. Harrigian, the air war commander, said in an interview from his headquarters in Qatar. “It’s improving. It’s still not at the level we’d like it to be.”
The air operation is a pivotal component of a military campaign that has cost $12.5 million a day in Iraq and Syria. The effort has destroyed hundreds of tanks, artillery pieces, military vehicles, command centers and fighting positions, and killed more than 50,000 fighters, according to American estimates. Since the air war began in late summer 2014, American and allied aircraft have conducted about 17,000 strikes in both countries.
The Islamic State has lost about half of the territory it seized in Iraq and Syria in 2014. But as ISIS loses ground in its physical caliphate, or religious state, the threat of hundreds of foreign fighters returning home and of the expansion of its virtual caliphate through social media is certain to accelerate, American and European officials say. That raises fears of more terrorist attacks in cities outside the Middle East.
For instance, the Islamic State has claimed responsibility for last week’s truck attack on a Christmas market in Berlin even though the links between the group and the main suspect, Anis Amri, a 24-year-old Tunisian, are not completely clear. After Mr. Amri’s death, the Islamic State released a video of him pledging allegiance to the group.
President Obama has vowed to deal the Islamic State crippling blows in Mosul and Raqqa before he leaves office. This month, he ordered 200 more American Special Operations forces to Syria to help local fighters advancing on Raqqa, nearly doubling the Pentagon’s boots on the ground there. Commanders are uncertain, however, about the level of support President-elect Donald J. Trump will maintain for rebel groups in Syria combating the Islamic State.
The military march on Raqqa is complicated by the predominant role played by Kurdish militia members, who make up a majority of the 45,000 fighters bearing down on the city. They are the most effective American partner against the Islamic State in Syria, providing logistics, command and control, and fierce fighting prowess. But the Kurdish fighters are viewed by Turkey — a pivotal American ally — as a terrorist threat.
These lingering diplomatic and military questions leave some congressional leaders voicing skepticism about a swift, decisive attack on the Islamic State capital. “It’s hard to see anything is imminent,” said Representative Adam Schiff of California, the senior Democrat on the House Intelligence Committee.
With a prewar population of about 220,000, Raqqa is about one-tenth the size of Mosul, but commanders still face the same challenges of waging an air war while minimizing risks to civilians in a congested city.
There are other reasons to go slow. Some Islamic State headquarters buildings have been spared attack for now so the Americans can monitor their communications and movements of their personnel in and out to learn more about the enemy operations, General Harrigian said.
Still, allied airstrikes have picked up as the Arab and Kurdish fighters have moved closer to the capital, and as commanders seek to pressure Mosul and Raqqa simultaneously. About 30 percent of the 1,300 strikes in and around Raqqa since the war began in 2014 have been conducted in the past three months.
“The pressure in Raqqa is bearing fruit as ISIL leaders come out of hiding, which allows us to kill them,” Brett H. McGurk, Mr. Obama’s envoy to the international coalition fighting the Islamic State, said this month.
Tracking the enemy’s ground movements falls largely to the crew of the Joint Stars plane, a 1960s-era, reconfigured Boeing 707 jetliner packed with sensitive electronics that is part of an eclectic and unsung mix of odd-shaped surveillance and reconnaissance aircraft with names like Compass Call and Rivet Joint. These planes suck up some enemy communications, jam others and help paint a picture of the Islamic State on the ground for American fighters and bombers to attack.
Bulging from the belly of the Joint Stars is a canoe-shaped, cloud-piercing radar that can see ground targets — and even some low-flying planes and helicopters — as far as 250 miles away on either side of the nearly windowless fuselage.
Aboard the plane, the crew of 19 Air Force and Army personnel — an unusual mix of active-duty and Georgia National Guard specialists — track clusters of dots on their screens that could represent groups of hostile fighters and their vehicles, friendly forces or just routine commercial traffic. Much depends on where they are and what time of day it is.
From its high-flying, wide-area perch, the radar can track moving vehicles; low, slow-flying aircraft; and smaller potential targets such as people, said Lt. Col. William B. Hartman, 39, of Irvine, Calif., the Joint Stars squadron commander. The operators on board can change the filters on their systems to show different-size targets or their direction of travel in different colors, all of which is relayed back to operations centers in Baghdad and Erbil, in northern Iraq, he said.
Flying from a base in the Persian Gulf, a typical Joint Stars mission over Iraq, Syria or Afghanistan can last about 11 or 12 hours. Crews pack snacks in their flight bags but also fire up the plane’s oven to prepare an in-flight order of chicken wings.
The Joint Stars is not equipped with cameras to identify specific images on the ground. When the crew members see something suspicious, they direct a Predator or Reaper drone to zoom in for a closer look. The Joint Stars is also valuable because a rotation of aircraft and surveillance crews can monitor a particular area for days, weeks or months, watching Islamic State activity to understand what the military calls the enemy’s “pattern of life.”
Islamic State fighters know from experience that they are being watched and often try to deceive the surveillance planes, hiding in schools or mosques or using camouflage. At one point, analysts said, ISIS even appeared to be trying to smuggle weapons strapped to the bellies of herds of sheep.
“They’re extremely smart,” Master Sgt. Caylon Kimball, 31, an airborne intelligence technician from Anadarko, Okla., said of the militants.
Several weeks ago, as the air campaign intensified against the Islamic State’s oil-production and distribution network, analysts noticed an intriguing development in the central Syrian desert, about 35 miles north of Palmyra.
Comparing months-old radar data from Joint Stars and other surveillance imagery with newer versions, analysts discovered that the Islamic State was moving much of its oil tanker truck fleet to an obscure area of sandy gullies, about 20 miles by 20 miles in size.
“They were trying to hide from us,” General Harrigian said. “They were adapting to what we were doing. They were going into the desert and just parking.”
For several more weeks, analysts watched the clandestine desert truck stop grow, wanting to ensure it was the Islamic State trucking fleet. Confident in that assessment, General Harrigian ordered an attack plan, code-named Olympus. In two waves of strikes — on Dec. 8 and 9 — about two dozen Air Force and Navy warplanes destroyed 188 of the trucks. Empty truck cabs were struck first to scare off drivers sleeping in their rigs, and General Harrigian said it appeared there were no civilian casualties.
Besides wiping out a sizable portion of the Islamic State’s tanker truck fleet and depriving the group of over $2 million in oil sales, commanders said the strike was also meant to cripple the enemy’s morale.
“There would be a larger strategic message we sent to them: Nice try. We found you,” General Harrigian said. “Keep trying to hide, we will hunt you down again.”