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."

But at a confirmation hearing, Hagel took on his critics and embraced Obama's policies.

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.

In 2011, he explained to the Financial Times what he meant.

"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."