As defense secretary, Ashton Carter would face a daunting pile of problems at home and abroad. And then there are the unforeseen crises, the ones that explode onto a Pentagon chief's agenda without warning.

Chuck Hagel, the man Carter would replace if confirmed, as expected, by the Senate, has noted that when he took the job in February 2013, he had no idea that U.S. troops would be back in a fractured Iraq or that the deadly Ebola virus in West Africa would require an urgent deployment of the 101st Airborne Division.

Even predictable challenges, such as pursuing and killing terrorists in the Middle East and Afghanistan, can be harder than they seemed on the outside, even for an experienced national security practitioner like the 60-year-old Carter. He served in the Pentagon under President Bill Clinton and was deputy defense secretary in 2011-2013.

Carter's confirmation hearings before the Senate Armed Services Committee are scheduled to begin Wednesday.

A sampling of the top issues facing the next defense secretary:

ISLAMIC STATE

Even though President Barack Obama expected the nation to be off a war footing by 2015, among the most vexing problems Carter would inherit is the war against the Islamic State group in Iraq and Syria.

The bombing of IS targets in Syria, which began in September, probably will continue well into Carter's tenure and maybe beyond. But he may face a more rapidly changing situation on the ground in Iraq, where the U.S. now has about 2,500 troops.

The Iraqi government wants to launch a major counteroffensive to regain lost territory, particularly the northern city of Mosul, but it is unclear whether Iraqi troops can succeed without U.S. soldiers by their side to call in airstrikes. Carter may have to decide in coming months whether to recommend to Obama that he authorize U.S. troops to perform that riskier, close-in combat role in support of the Iraqis.

Carter also would manage — and assess the effectiveness of — a program designed to train members of the moderate Syrian opposition.

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THE BUDGET

The looming automatic budget cuts, known as sequestration, will be one of Carter's priorities because everything the military does is based on having enough money to pay for troops, equipment, weapons and training. Unless Congress takes action, the steep cuts initially approved in 2011 would be reinstated.

Defense and military leaders have insisted that deep cuts will require more reductions in the size of the force, particularly the Army, and make it more difficult to keep troops prepared to respond to threats or upheaval around the world.

Carter's main job will be as the top salesman leading the charge on Capitol Hill and persuading lawmakers not only to reverse the cuts but also bolster Pentagon spending.

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AFGHANISTAN

Obama decreed that America's combat mission in Afghanistan is over, but there are more than 10,500 U.S. troops on the ground and many are still conducing counterterrorism operations against the Taliban and other insurgents.

American and coalition forces continue to train and advise the Afghan military. Obama has said that the U.S. can continue to provide ground and air support to the Afghan forces when needed.

Carter, however, will have to deal with nagging questions about the pace of the U.S. withdrawal from Afghanistan, which under current plans would have all U.S. troops out by the end of 2016. Afghan officials are worried about the reduction in U.S. troop support.

U.S. military commanders say they will wait until after this summer's fighting season to decide if they should request any changes to the current drawdown. Any change to the pace could be seen as Obama reneging on his promise to end the war, making such a request politically tricky for Carter.

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RUSSIA

The U.S. is relying on NATO partners to help pressure Russia to relent in its support of anti-government rebels in eastern Ukraine — a problem that aligns with Carter's long history of advocating for closer NATO ties to Ukraine.

Carter would be expected to weigh in on the question of whether to expand U.S. assistance for Ukraine to include weaponry.

Carter's background also fits another Russia problem: Moscow's reluctance to continue with a decades-long U.S. program to help secure surplus Russian nuclear materials to ensure they do not fall into terrorists' hands. Carter has focused on the problem of "loose nukes" in Russia since the collapse of the Soviet Union in 1991.

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CHINA

Improving defense relations with China is likely to rank as a Carter priority, in part because of tensions over Beijing's growing military might, regional influence and expanding cyberwarfare capability. Carter will have to key an eye on the other leading defense challenge in Asia: North Korea's nuclear weapons program.

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HEALTH OF THE FORCE

After more than a dozen years at war, America's service members have battled more than enemy insurgents. At home, suicides, sexual assaults, traumatic brain injuries and post-traumatic stress all increased as the wars dragged on. Both suicides and reported sexual assaults increased last year, compared with 2013.

The Pentagon sees the increase in reported sexual assaults as a positive sign that victims are more willing to come forward. But the military services continue to struggle to reduce assaults while also protecting victims are insuring they get proper care. It will be up to Carter to continue to pressure the services to make progress.

He also will be the final arbiter when the military services come forward later this year to say what combat jobs should not be opened to women. While thousands of front-line jobs are now open to women, many of the more difficult infantry, armor and commando jobs are still being reviewed and debated.