Outgoing Joint Chiefs Chairman Martin Dempsey Reflects on His Toughest Day on the Job

— -- Gen. Martin Dempsey has seen it all in his four years as chairman of the Joint Chiefs of Staff, from the devastation in Iraq and Syria to today’s refugee crisis in Europe.

But he said the toughest day in his military career was when he stood at Dover Air Force Base and met the body of one of his son’s West Point classmates who had been killed while serving in Afghanistan. He described the moment as “heart-wrenching.”

“A young man named Tom Kennedy. Just a terrific kid,” Dempsey told ABC News’ Chief Global Affairs Correspondent Martha Raddatz in an exclusive interview. “Those kinds of days are unimaginable.”

Ahead of his final day as chairman later this month, Dempsey also described one of the best days of his career as one that happened recently -- watching 1st Lt. Kristen Griest and Capt. Shaye Haver make history as the first female graduates of the elite Army Rangers School.

Dempsey called his conversation with them "one of the most uplifting, inspiring phone calls" he has made as chairman.

"They were two West Pointers and I couldn’t have been more proud as a West Pointer to have that be the case,” said Dempsey, who graduated from the military academy in 1974 when it was still an all-male institution. “What it reminded me is that our profession is built on trust and if they made it through that course and coming out the other end had earned the trust of their male counterparts, that’s profound.”

Appointed by President Obama as the nation’s top military officer in 2011, Dempsey has since advocated for more thoughtfulness in deploying U.S. military force.

Despite some critics calling him a reluctant warrior, Dempsey said he was satisfied with a legacy of cautiousness.

“A military leader should always understand, of all human endeavors ... the one that’s the most unpredictable and the most costly is warfare,” he said.

Upon retiring from his position, Dempsey plans on keeping a box that holds the names of the service members who died under his command.

“It says, ‘Make it matter,’” he said. “That’s my legacy.”

Dempsey steps down as chairman at the end of September. Marine Gen. Joseph Dunford is set to succeed him.