WASHINGTON - When he started working on defense issues, Ashton Carter said Wednesday, there was just one big worry: the Cold War and its threat of nuclear disaster.

"Those were the good old days," said Sen. James M. Inhofe (R., Okla.).

Now, as Carter stands poised to become the civilian head of the U.S. military, the Abington High School graduate faces a much more complex tangle.

"We are in a time where the number and severity of risks is not something I've seen before in my life," Carter said during the first hearing on his nomination to become secretary of defense.

His point was illustrated by the topics covered with the Senate Armed Services Committee: the Islamic State, Iran's nuclear ambitions, Afghanistan, a rising China, President Obama's "pivot" to Asia, and cybersecurity, not to mention domestic issues such as sexual harassment in the armed forces, budget cuts, military cost overruns, and the debate over prisons at Guantanamo.

"Even in the best of times," Chairman John McCain (R., Ariz.) said as the hearing opened, "the position for which you have been nominated is one of the most challenging in government."

The committee's top Democrat, Sen. Jack Reed of Rhode Island, noted that Carter would oversee 1.3 million active-duty military members, 820,000 in the reserve or National Guard, and 773,000 civilians. The Defense Department spends about $600 billion each year, more than any other area of government aside from Social Security, Medicare, and Medicaid.

Carter, 60, is a Yale- and Oxford-educated physicist who has never been in the military but has spent more than 30 years moving between academia and the Defense Department.

Born in Philadelphia, raised in the Montgomery County suburbs, and widely described as blunt and brilliant, Carter earned honors in medieval history and physics at Yale.

Arms for Ukraine

Carter was the Defense Department's second-ranking official as recently as 2013 and before that led its vast purchasing efforts.

Known as "Ash," he has served under 11 defense secretaries and has won praise from senators in both parties, so he is expected to easily win confirmation.

But lawmakers used Wednesday's hearing to prod him on the conflagrations around the globe, and some took aim at what they see as feckless foreign policy by Obama.

Carter broke ground when he said he was "very much inclined" to provide lethal arms to Ukrainians fighting pro-Russian separatists, a move that could bolster their forces but risk escalation of the conflict.

Carter said the United States made a commitment to Ukraine to ensure its independence as part of a 1994 deal persuading it to give up its nuclear weapons.

"That is at stake today," he testified.

Islamic State

He described the administration's plan for fighting the Islamic State, saying a "lasting defeat" of the group hinged on bolstering partners on the ground.

Carter said the administration aimed to strengthen government forces in Iraq, and work with moderate opposition fighters and regional powers to combat the Islamic State in Syria.

"It doesn't sound like a strategy to me," said McCain. "It sounds like a series of goals."

McCain and some Democrats also criticized the administration's plans to pull troops out of Afghanistan by the end of 2016.

"It's a plan," Carter said. "If I'm confirmed and I ascertain as the years go by that we need to change that plan, I will recommend those changes to the president."

Sen. Tim Kaine (D. Va.) said, "I really hope we have a conditions-based strategy and not a calendar-based strategy."

To other senators' questions, Carter vowed not to let political pressure influence decisions on how and when to transfer prisoners out of Guantanamo, and to take on sexual assault in the armed services.

"It's particularly offensive in the military community," Carter said, "because the military ethos is one of honor and trust."

McCain, citing scathing critiques from two of Obama's previous defense secretaries, warned that Carter may face "micromanagement" from the White House and said he had concern about the influence Carter would actually have.

Carter pledged to give Obama "my most candid strategic advice" and, in a hint that he would resist political pressure, said, "I will be a stickler for the chain of command."

On spending, he urged lawmakers to help find a way "out of the wilderness of sequester," the automatic budget cuts that have slashed domestic and defense programs.

But he also cast a skeptical eye on Pentagon cost overruns and a lack of accountability for waste.

"Every company, state, and city in the country has had to 'lean' itself out in recent years, and it should be no different for the Pentagon," Carter said.

Add that to a long list of challenges ahead.