Story highlights Chuck Hagel arrives in Kabul, where he says he'll meet with Karzai and thank the troops

Hagel was previously highly critical of the surge in Afghanistan and the war in Iraq

He said in late the 2000s: "I'm not sure we know what ... we are doing in Afghanistan"

His own experiences in Vietnam is likely to inform his style in the job

The first Vietnam veteran to be U.S. defense secretary is spending his first overseas trip on the job thanking soldiers and Marines.

Around 11 a.m. ET Friday, Chuck Hagel touched down in Kabul, Afghanistan.

"I think it's always important when new leadership comes in to any office in our national security organization, that we recognize the people who make it all possible and who are the ones on the front lines securing this country," he said while en route.

A one-page letter from him will be handed out to troops.

But Hagel is also there to "better understand what's going on" in the 11-year war -- America's longest. That will help him "better advise the president, and to do my job as well as I can, to make my own assessment and listen to our commanders," he said.

Hagel told reporters that he's known Afghan President Hamid Karzai for 11 years and he expects to talk with him about many topics, including Karzai's recent restrictions on U.S. Special Operations Forces.

"We're still at war in Afghanistan," he said, although it was never the United States' intention to stay indefinitely.

Many in Congress, including several high-ranking members of his Republican Party, opposed Hagel's nomination; the final vote in the Senate was 58-41.

Besides not liking his past comments about Israel and Iran , they bristled at his comments over the years about Iraq and Afghanistan, some of which came after Hagel went with Barack Obama, then an Illinois senator, to Afghanistan, Iraq, Jordan and Kuwait in 2008.

In 2009, Hagel opposed Obama's decision as president to send another 30,000 troops into Afghanistan.

"I think we're marking time as we slaughter more young people," he told the National Journal. "I'm not sure we know what the hell we are doing in Afghanistan."

En route to Afghanistan Friday, he said that the United States will remain committed to the country as U.S. troops are being moved out and Afghan troops take the lead.

"We're still at war," he said. "I think most Americans, the Congress, the media understand that.

"And that fact remains we have 66,000-68,000 troops still at war in a combat zone. And so that reality is there. ... We'll stay focused on that. The president, I think has been very clear on that."

Responding to a reporter's question about whether he draws comparisons between Vietnam and Afghanistan, Hagel replied, "There are always parallels to any war. The only thing I would say is the world we live in today is so complicated. And we have to factor that into our policies and everything that we do."

Hagel has tried over the years to offer context and nuance to his past statements about both Iraq and Afghanistan, hoping that he might be better understood.

"I disagreed with President Obama, his decision to surge in Afghanistan, as I did with President Bush on the surge in Iraq.

"It wasn't a matter of could we win at that moment. Of course, no force in the world can stand the sophisticated power of American military."

The Obama administration now plans to pull combat troops out of Afghanistan by 2014, replacing them with a training mission to advise Afghan forces, steps Hagel will oversee if confirmed.

The Financial Times interview gave insight into how Hagel, 66, might approach his new job.

There will always be dictators and hostilities in certain regions, he said, but the United States must continue "working with our partners, working with other countries, with other regional powers, working through the United Nations.

"That's the way to approach these great imponderables -- difficult, complicated situations," he told the magazine, "because then you ask yourself, well, what are my options?"

In January , Obama and Karzai agreed that this spring, Afghan forces will take primary control of the country.

U.S. officials have said anywhere between zero to 9,000 U.S. forces could remain in Afghanistan past 2014.

During a hearing on Hagel's nomination in January, he spoke about Afghanistan.

"As to what kind of a force structure should eventually be in place by the Afghans, I don't know enough about the specifics to give you a good answer other than to say that I think that has to be a decision that is made certainly with the president of Afghanistan," he said.

Talking with Karzai will inform "what we can do to continue to support and train and protect our interests within the scope of our ability to do that," he said. "Obviously the immunity for our troops is an issue, which was an issue with Iraq. All those considerations will be important and will be made."

Hagel also said during that hearing that going to war in Iraq took the U.S. focus off Afghanistan.

The defense secretary knows from personal experience that good strategy must consider the human toll of war.

Before he became a two-term senator from Nebraska, a Georgetown professor or the head of a D.C. think tank , Hagel volunteered to join the Army and go to Vietnam.

As a sergeant, he was twice wounded and fought alongside his younger brother Tom

Chuck Hagel earned two Purple Hearts. His brother patched up his wounds when he took shrapnel in the chest while on patrol, and Chuck Hagel saved his brother's life after Tom Hagel was wounded.

"I will do all I can to prevent war," he later told his biographer.

But don't misunderstand, Hagel has said.

"Not that I'm a pacifist -- I'm a hard-edged realist, I understand the world as it is -- but war is a terrible thing," he is quoted in the 2006 biography, "Chuck Hagel: Moving Forward."

"There's no glory," he said of war, "only suffering."