It's been 47 years since Chuck Hagel patrolled the thick jungles of Vietnam as an Army squad leader.

But even now, in his pristine office at the Pentagon overlooking the Potomac River, the defense secretary still can vividly recall the night in 1968 when his armored personnel carrier hit a 500-pound mine.

"I remember that night like it was yesterday," Hagel said in a recent interview in his office.

Hagel was a 21-year-old Army sergeant when he was hit in the chest with shrapnel, his face burned from a mine blast, his eardrums blown out. After a gunfight with Viet Cong troops, he recalls, he and other soldiers who were in "pretty bad shape" waited for a medevac helicopter.

"My brother was with me as well. They dusted us off to a field hospital. ... It has been seared into my mind," he said.

"We're products of our environments. We're products of our experiences. Certainly that experience in war in Vietnam in 1968 ... the violence, the horrors, the suffering that I saw, yes, it conditioned me."

"But," Hagel added after a brief pause, "I also recognize the realities of the world that we live in. The instrument of our military is required, unfortunately, and history is rather replete on that point. Maybe someday they will not be required, but they are.

"Then the question becomes the use of that instrument of power and force. Using it wisely, using it carefully, and using it judiciously. We have not always done that and I have been very vocal on that point," he said.

Hagel will soon step down after two years on the job, reportedly amid tension with the White House's inner circle over the U.S. response to burgeoning crises that include the rise of the Islamic State group in Iraq, new Russian aggression in Europe and the Syrian civil war.

Hagel 's experience at the Pentagon has been unique in several ways. He is the first Vietnam veteran to hold the powerful post, and the first who served as a noncommissioned officer. And unlike several of his predecessors, he did not arrive with extensive experience as a national security professional, but rather with a background working with veterans and the military community — he was the number two man at the Veterans Affairs Department in the 1980s and later served as the CEO of the USO.

His legacy will he shaped in part by his caution regarding the use of military force.

"I do believe that you always have to ask the tough questions ... like, once you take this action, that is not the end of that action ... because there will be consequences that will come from this action. Now where do you want to go? What happens next? Where do you want this to end up? Those are the questions that you ask. This also applies to inaction. If you take no action, what are the consequences?" Hagel said.

His supporters say that approach is measured and prudent; he is careful to consider the full impact of military strikes and troop deployments. But his critics say he has been indecisive, hesitant to exercise America's massive military might and uncomfortable with the tough choices required of the leader of the Defense Department.

Hagel is not the first defense secretary to urge caution in using military force. In 2011, former Defense Secretary Robert Gates memorably noted that "any future defense secretary who advises the president to again send a big American land army into Asia or into the Middle East or Africa should have his head examined."

But such views have been more central to Hagel's leadership. His belief in the limits of military action comes at a time when the Pentagon is grappling to adjust to new limits on its own size and budget. In that regard, Hagel has helped map out a long-term plan for a more modest military that can be sustained under the defense spending limits that Republicans and Democrats approved and appear unwilling to change.

Shortly after taking over the Pentagon's top job in 2013, Hagel launched a "Strategic Choices and Management Review," which floated once-unthinkable proposals such as cutting the Army to 420,000 soldiers and scaling back the size of the Navy aircraft carrier fleet.

"I am the first secretary of defense who has actually had to deal with sequestration," Hagel said in the interview, referring to the legal mechanism that imposed across-the-board cuts in military spending in 2013. "I am the first secretary of defense who actually had to put together a budget, a real budget, a practical budget with the law of sequestration hanging over us.

"We had not prepared a budget for this reality," he said.

Some experts say Hagel's caution regarding the use of military force tracks with the broader military culture of the moment: Troops are weary after 13 years of war, the officer corps is questioning many decisions that guided the wars in Iraq and Afghanistan and senior Pentagon officials are dealing with tough budget cuts from Capitol Hill.

"I think many [in the] military have been kind of skeptical of going back into Iraq with a large ground force," said Richard Kohn, who teaches military history at the University of North Carolina at Chapel Hill. "They are not nearly as bellicose as some of the hawks in the Republican Party because they recognize their limitations, both in funding and capability."

Iraq creep

In 2008, when Hagel was a Republican senator from Nebraska, he bucked his party's leadership to vocally oppose the "surge" that sent thousands of additional U.S. troops into Iraq.

He visited the White House frequently last summer as the Obama administration and senior military officials were formulating a strategy and response to the rise of a new wave of Islamic extremists in Iraq. Hagel's role behind closed doors is unclear, but it is widely believed that he brought to the table a series of questions and concerns about pursuing a military solution to the crisis.

There are now 3,100 U.S. troops authorized to serve in Iraq, and an advise-and-assist mission to support Iraqi troops is underway. The U.S. and its coalition partners have dropped more than 1,700 bombs on Iraq and Syria since August.

Hagel said he worries about mission creep as he prepares to step down and hand over these policies and operations to his successor.

"I think any secretary of defense has to always be on guard that we do not inadvertently sometimes drift into a more accelerated use ... of our military," he said. "It is always a concern for me and I suspect any secretary of defense.

"Those of us responsible for this institution have to always be very clear with our leaders in Congress, our commander in chief ... What is the mission? What are the objectives? How best can we fulfill that mission? And it is easy to drift into other missions."

He said he believes those questions are especially important "at the moment of crisis ... when there is an expectation from the American people, from media, the Congress: 'What are you going to do?' Well, the military answer is not always the right answer," he said.

With his next comment, Hagel once again acknowledged that his perspective on such questions is heavily shaped by his own experience in Vietnam:

"I learned as I walked through this 12 months of war in 1968 ... you cannot impose your will, you cannot impose your values, you cannot impose your standards, your institutions on other societies in other countries. It has never worked. Never will work.

"People want freedom, want rights. But they also want respect and they want dignity. That means respecting other's cultures and other's religions and ways of life. … Nations, cultures, people come from different histories and they have to work through their own societal issues to get to where they want to be," Hagel said.

That's why, he said, the U.S. and its military are now seeking to forge partnerships with local militaries around the world.

16,000 casualties

Hagel usually shies away from speaking publicly about his time in Vietnam. According to several people who work closely with him, he consciously avoids the appearance that he is exploiting his experience for political gain.

When asked about his deployment, Hagel notes the total casualties that America suffered during his year there.

"We sent home 16,000 young men — and some women, not many, but some — who had been killed in that war. That is almost unimaginable today to think about that," he said.

"I saw the suffering of our own troops. I saw the suffering of the Vietnamese people. I saw terrible things which war always produces. So I have tried to do what I could within the jobs I had and the positions I had, to help influence that ... influence a world that maybe someday it will not require that use of force.

"But," he added, "we live in the world that we live in."