Defense Secretary James Mattis walks into the Pentagon in Washington, D.C., April 11, 2018. Tech Sgt. Vernon Young Jr. | Department of Defense

ABOARD A US MILITARY AIRCRAFT — Under the cover of predawn hours, Secretary of Defense James Mattis disembarked a 20-hour flight from the Middle East at Andrews Air Force Base and headed straight to the Pentagon. It was the morning of his 68th birthday and by all accounts it was just another day in the Secretary's demanding schedule. Earlier in the week, Mattis and his staff began a five-day trip with stops in San Diego, New Delhi, Kabul and Abu Dhabi. He would clock more than 22,000 flight miles and 42 hours aboard planes, helicopters, and cargo aircraft. His days, despite jumping between multiple time zones, began at 4 a.m. and were filled with engagements. Carrying his own luggage and arriving early to the flight line are habits learned by Mattis' team, who work to stay two steps ahead of the exacting Marine Corps general. Since taking the highest office in the Pentagon, Mattis has visited nearly 60 countries.

Secretary James Mattis climbs up the steps as he boards a US Air Force C-17. Jonathan Ernst /AFP/Getty

But back in Washington, his work abroad was largely eclipsed by a report claiming that he recently likened Trump's intellect to that of "a fifth- or sixth-grader." Passages from journalist Bob Woodward's book, "Fear: Trump in the White House," described a contentious relationship between Mattis and Trump, according to a Washington Post article. The revelations emerged nearly three hours into a 22-hour flight that departed San Diego for New Delhi. While en route, Mattis released a statement vehemently denying the accusations. "The contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence," Mattis said in a statement. "While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility."

Secretary of Defense James Mattis speaks with reporters while on board the E-4B aircraft. Tech Sgt. Vernon Young Jr. | Department of Defense

Hours later another Washington Post report said Trump was displeased with his Pentagon chief and was mulling over a replacement. The news report pre-empted high-level talks between Mattis, Secretary of State Mike Pompeo and their Indian counterparts. While in New Delhi, Mattis signed a security communications agreement, an item reserved for the most trusted defense partners of the United States. The Communications Compatibility and Security Agreement, or COMCASA, strengthens the defense relationship between the world's two largest democracies by allowing the U.S. to transfer high-tech military equipment to India. Mattis' schedule in New Delhi also included dinner with Prime Minister Narendra Modi.

U.S. Secretary of Defense James Mattis and Indian Defense Minister Nirmala Sitharaman sign the Communications Compatibility and Security Agreement( COMCASA in, New Delhi, India, Sept. 6, 2018. Lisa Ferdinando | CNBC

After two nights in New Delhi, enough to adjust to the 12-hour time difference, Mattis secretly boarded a C-17 Globemaster and then a CH-47 Chinook to Kabul, Afghanistan. The unannounced trip was his fourth visit to Afghanistan since becoming Defense Secretary. He was accompanied by Chairman of the Joint Chiefs of Staff Marine General Joseph Dunford, who earlier in the week visited Islamabad with Pompeo to hold talks with Pakistan's new government. There are approximately 14,000 Americans in Afghanistan.

Defense Secretary James Mattis arrives in Kabul, Afghanistan Army Sgt. Amber I. Smith | Department of Defense

Upon arrival, Mattis met with U.S. Army Gen. Scott Miller, the new commander of NATO troops in Afghanistan. Mattis' visit comes amid recent attacks. Four days prior, a U.S. service member was killed and another wounded in "an apparent insider attack" in eastern Afghanistan, according to a statement from the Resolute Support, the NATO-led coalition in Afghanistan. A month prior, the Islamic State group, also known as ISIS, claimed responsibility for a bombing that killed 48 people at a college preparatory academy in Kabul. Mattis also met with Afghan President Ashraf Ghani at the presidential palace to discuss progress to end the 17-year-old war in Afghanistan, which has become America's longest conflict.

U.S. Secretary of Defense James N. Mattis arrives at the Afghan Presidential Office building, Kabul, Afghanistan, Sept. 7, 2018. Lisa Ferdinando | Department of Defense