WASHINGTON >> For only the second time since 9/11, America’s defense secretary didn’t visit U.S. troops in a war zone during December, breaking a long-standing tradition of personally and publicly thanking service members in combat who are separated from their families during the holiday season.

Pentagon boss Jim Mattis, who spent more than four decades in the Marine Corps and deployed to Iraq and Afghanistan, made a five-day trip through the Middle East in early December. He stopped in Kuwait and Pakistan — countries adjacent to Iraq and Afghanistan — but didn’t cross the borders to see troops at war in either country. Last week, he visited troops in Guantanamo Bay, Cuba, and at military bases in Florida, North Carolina and South Carolina, wishing them holiday cheer.

It has been 15 years since a U.S. defense chief didn’t travel to a war zone during the festive season. And the only time a holiday visit was skipped since Americans began fighting in Afghanistan was in December 2002. That year, then-Secretary Donald Rumsfeld went to a command post in Qatar that would be used a few months later to coordinate the launch of the Iraq war.

Asked recently why he wasn’t going to Iraq or Afghanistan, Mattis said he didn’t want to discuss his travel. “I carry out my duties to the best of my ability,” said Mattis, who visited Iraq and Afghanistan earlier this year.

Dana White, his chief spokeswoman, said the secretary “wanted the troops to enjoy their holiday uninterrupted. He is keenly aware of the logistical challenges of a senior leader visit, especially in a war zone.”

Defense secretary trips historically have been aimed at boosting troop morale, letting service members know that senior leaders and the U.S. public recognize their sacrifice. And generals who have chaired the Joint Chiefs of Staff have routinely done their own December trips to war zones, taking celebrities on their flights as part of a USO entertainment tour.

It is less of a tradition for U.S. presidents to make December visits to conflict zones. Such trips require much greater logistical and security planning.

President George W. Bush visited Afghanistan twice and Iraq four times, including a secret Thanksgiving voyage to Baghdad in 2003 and a trip to both nations’ capitals in December 2008. President Barack Obama flew to Iraq once as commander in chief and four times to Afghanistan. Only a December 2010 trip came during the holidays.

President Donald Trump hasn’t yet gone to the war front, but Vice President Mike Pence flew to Afghanistan last week.

Less than three months after U.S. troops invaded Afghanistan, Rumsfeld flew into Bagram Air Base under extraordinarily high security, telling service members the World Trade Center “is still burning as we sit here, they’re still bringing bodies out.”

He said he made the trip to talk face-to-face with “real people who are doing real things that are part of our plan.”

In two subsequent years, Rumsfeld went to Afghanistan and Iraq on Christmas Eve, mingling with troops and donning an apron to serve them holiday dinner. He fielded questions or complaints, too. In one such December troop talk, when a soldier asked Rumsfeld why troops went into battle in Iraq badly equipped, the secretary gave a now-famous response: “You go to the war with the Army you have … not the Army you might want or wish to have at a later time.”

Rumsfeld’s successor, Robert Gates, maintained the tradition, traveling to the war zone around the holidays during each of his five years in office. His first trip was on Dec. 20, 2006, two days after taking the job.

Like his predecessor, Gates ended his troop talks with the traditional lineup for handshakes, photos and the much-desired commemorative coin. The coins — different for each secretary or military commander and emblazoned with their names or unit designs — are ceremonial gifts that young service members embrace. Many collect them or use them to get free drinks in bars. One game stipulates the service member with the highest-ranking coin wins.

Secretaries Leon Panetta, Chuck Hagel and Ash Carter followed suit, trailed by a military aide carrying the coins that they often handed out to hundreds after each event.

“Christmastime’s coming up and from our family to your families, thank you,” Carter, Obama’s final Pentagon boss, told troops in Afghanistan during a December 2015 visit. “You’re not with them, you’re here. We don’t take that for granted.”

Mattis rarely gives public troop talks, in any season. Usually he meets privately with small groups of service members. And he has declined to hand out coins.

While the December visits often have been promoted as a way to thank troops, they have sometimes brought a special Christmas gift.

In 2010, Gates gave four soldiers a ride home from Afghanistan on his military plane. Beyond the faster, more comfortable flight, they got a one-night layover in the luxurious Emirates Palace hotel in Abu Dhabi, where Gates stopped to meet local leaders.

“The next time you’re in touch with your families, I hope you’d let them know, whether it’s email or a phone call or whatever, just pass along to them my personal thanks to them for their support to you and their patience with all of us,” he told troops at Forward Operating Base Howz-e-Madad, Afghanistan, that December.