After nearly an hour's flight north from Baghdad, a cavernous C-130 military cargo plane touches down. Aboard are reporters, Pentagon officials and the man who has occasioned this trip, Defense Secretary Ashton Carter.

The plane taxis along an airstrip that as recently as July was controlled — and then largely destroyed — by Islamic State fighters.

This is the Qayyarah Airfield West, just 30 miles south of Mosul.

When Islamic State fighters attacked Mosul more than two years ago, soldiers guarding Iraq's second-largest city fled in disarray. Now, with an assist from the U.S., Iraq's security forces are two months into a fierce battle to retake Mosul.

This base is now firmly under U.S. and Iraqi control, and it is a key staging site for the assault on Mosul.

The late afternoon sun is low, the temperature in the 30s and the air brisk as we follow crushed rock trails over fine, tan dust. The high concrete blast walls lining the base's perimeter cannot shield it from enemy mortars. So everyone piles into hulking, explosion-resistant vehicles called MaxxPros as we head over to what's known as "the life support area."

Our destination is essentially a big tent covering a bunker for American troops. They snap to attention as Carter enters.

"Happy holidays to you," is the Pentagon chief's greeting to these soldiers from the 101st Airborne Division. "This is not the most wonderful place to spend the holidays, so we wanted to make sure you knew we appreciate it."

The troops' bunks — made of foot-thick concrete slabs — are festooned with photos and football banners.

"You wouldn't be here if I didn't need you to be here," Carter tells the fresh-faced soldiers. "I need you to be here because we need to protect our people. And to do that we need to defeat ISIL, and the Iraqis could no way ever do it by themselves. And so it's a historic necessity."

Carter has also come here to bestow medals for bravery — but not for the 500 or so Americans at this fire base.

In a shed that serves as the Combined Joint Operations Command, the defense secretary pins medals on eight Iraqi soldiers — elite troops who have been doing deadly block-to-block fighting in eastern Mosul.

"These troops have braved snipers and IEDs and mortars, suicide cars, truck bombs, and more, but they have shown true bravery in battle," Carter tells a small crowd of U.S. and Iraqi officers. "Because of that bravery and determination, I'm confident that they will prevail."

After the U.S. invaded Iraq nearly 14 years ago, American troops did most of the fighting. But during the Obama administration, their role has been more to train, advise and assist. Iraqis now do most of the fighting and dying.

Pentagon officials say the 5,000 or so U.S. troops deployed to Iraq are there mainly for noncombat duties, such as training local forces or targeting airstrikes.

"It's a team effort, so if they get intelligence, they'll come to us with that information and then we'll talk about it," says Marine Capt. Lou Murillo, describing his work identifying potential targets in collaboration with Iraqi forces at this fire base. "If we have intelligence, we'll talk with them and make sure that we are both included on the approval process for it. And then we will send a request up to our higher headquarters and they process the targets from there."

"There" is usually Baghdad, where top U.S. commanders are based — and where Iraqi officials must sign off on strikes as well. For the Americans, there are strict White House-imposed limits on the number of civilians who might possibly be killed in an airstrike — usually no more than 10. That complicates carrying out airstrikes in the still densely populated neighborhoods of Mosul, where residents are reported to be used by Islamic State fighters as human shields.

Senior American defense officials say Iraqi officials have also blocked targets, including Mosul's central bridge across the Tigris River. They want that bridge kept intact for when Mosul is retaken.

For troops on this fire base, the retaking of Mosul is more a question of when than if.

"It's been goin' pretty good. We keep on moving forward, hopefully Mosul before the beginning of the year," says Army Sgt. Brian O'Toole.

Carter is certain Mosul will be retaken, though he is not saying when.

"We all want to get this done as soon as possible," he says, "but in a war, that's what you have to expect. It's gonna be a tough fight."

So tough, in fact, that there's concernthat Iraq's American-trained Golden Brigade, which has done the bulk of the block-to-block fighting in Mosul, could soon be crippled by a 50 percent casualty rate.

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