KANEOHE BAY, Hawaii — President Barack Obama marked the end of more than a decade of combat in Afghanistan by paying tribute to America's military, telling troops on Christmas Day that their sacrifices have allowed for a more peaceful, prosperous world to emerge out of the ashes of 9/11.

At an oceanfront Marine Corps base in Hawaii, Obama told troops that although tough challenges remain for the U.S. military in hotspots such as Iraq and West Africa, the world as a whole is better off because American troops put country first and served with distinction. He said Americans and their president could not be more thankful.

"Because of the extraordinary service of the men and women in the American armed forces, Afghanistan has a chance to rebuild its own country," Obama said to applause from Marines and their families. "We are safer. It's not going to be a source of terrorist attacks again."

Thirteen years and $1 trillion later, the U.S. is preparing to pull the vast majority of its combat troops out of Afghanistan by year's end, as the U.S. and its partners seek to turn the page on a bloody chapter that started the day that al-Qaida militants struck American soil on Sept. 11, 2011. From a peak of 140,000 troops in 2010, the U.S. and NATO plan to leave about 13,500 behind for training and battlefield support.

Although there are reasons for cautious optimism, including a new Afghan president whose seriousness of effort has inspired U.S. confidence, the broader picture still looks grim.

The U.S. is shifting to a supporting role after the bloodiest year in Afghanistan since the 2001 U.S.-led invasion. Civilian casualties this year are on track to hit 10,000, and some 5,000 Afghan forces were also killed in 2014, a figure that has escalated as the country took on a greater role in its own security. Insurgents have seized territory across the country, raising fears that Islamic militants will successfully exploit the security vacuum formed as the U.S. pulls out.

About 2,200 U.S. troops were killed in Afghanistan during the past 13 years in a war that cost the U.S. $1 trillion, plus another $100 billion for reconstruction. A celebratory cheer of "hooah" rang out from the hundreds of troops in attendance when Obama affirmed that the combat mission was finally ending.

"We still have some very difficult missions around the world — including in Iraq," Obama said. But, he added, "the world is better, it's safer, it's more peaceful, it's more prosperous and our homeland protected because of you."

The US military has slowly been stockpiling equipment and gear coming out of the war in Afghanistan in a Kuwait depot, ultimately destined for Iraq ahead of a possible spring offensive against the Islamic State.

The warehouse in Kuwait’s Shuhaiba port houses over 3,000 mine-resistant, ambush-protected vehicles and electronic gear, according to the news site US News.

“From June to December, we’ve worked a lot on moving items into Kuwait,” said US Air Force Maj. Gen. Rowayne “Wayne” Schatz, the director of operations and plans for US Transportation Command. “The army is holding the gear there, and it has room to hold it, as the mission fleshes out,” he told US News.

The US-led coalition is reportedly planning a major operation this spring aimed at helping Iraqi troops and Kurdish peshmerga forces retake land conquered by the Islamic State in a blitz that started in June.

According to the report, US military authorities are set to focus on wresting back Anbar province and the city of Mosul.

Earlier this month, the outgoing Secretary of Defense Chuck Hagel signed the order for the first group of US troops to go to Iraq as part of the administration’s recent decision to deploy 1,500 more American forces to the country. The troops are to advise and train Iraqi forces.

President Barack Obama spoke with Iraq Prime Minister Haider al-Abadi, reiterating to him the US commitment to train and assist the Iraqi military, provide weapons and equipment, and continue to launch airstrikes against the militants.

The top US commander for the mission in Iraq and Syria said it will take at least three years to build the capabilities of the Iraqi military.

Army Lt. Gen. James Terry, who is leading the US campaign to defeat Islamic State militants in Syria and Iraq, said the challenge is to get Iraqi units trained and back into the fight so they can plan operations to regain contested areas such as Mosul.

He said that while there has been progress in halting the militants’ charge across Iraq, “I think what we must do, especially inside of Iraq, is continue to build those (Iraqi) capabilities. I think you’re at least talking a minimum of three years.”

(Reuters) - The U.S. government is preparing to boost the number of private contractors in Iraq as part of President Barack Obama's growing effort to beat back Islamic State militants threatening the Baghdad government, a senior U.S. official said.

How many contractors will deploy to Iraq - beyond the roughly 1,800 now working there for the U.S. State Department - will depend in part, the official said, on how widely dispersed U.S. troops advising Iraqi security forces are, and how far they are from U.S. diplomatic facilities.

Still, the preparations to increase the number of contractors - who can be responsible for everything from security to vehicle repair and food service - underscores Obama's growing commitment in Iraq. When U.S. troops and diplomats venture into war zones, contractors tend to follow, doing jobs once handled by the military itself.

"It is certain that there will have to be some number of contractors brought in for additional support," said the senior U.S. official, speaking on condition of anonymity.

After Islamic State seized large swaths of Iraqi territory and the major city of Mosul in June, Obama ordered U.S. troops back to Iraq. Last month, he authorized roughly doubling the number of troops, who will be in non-combat roles, to 3,100, but is keen not to let the troop commitment grow too much.

There are now about 1,750 U.S. troops in Iraq, and U.S. Defense Secretary Chuck Hagel last week ordered deployment of an additional 1,300.

The U.S. military’s reliance on civilians was on display during Hagel's trip to Baghdad this month, when he and his delegation were flown over the Iraqi capital in helicopters operated by State Department contractors.

The problem, the senior U.S. official said, is that as U.S. troops continue flowing into Iraq, the State Department's contractor ranks will no longer be able to support the needs of both diplomats and troops.

After declining since late 2011, State Department contractor numbers in Iraq have risen slightly, by less than 5 percent, since June, a State Department spokesman said.