"Storm clouds are gathering" over the Korean Peninsula, Defense Secretary Jim Mattis declared Dec. 22.
And as diplomats try to resolve the nuclear standoff, he told soldiers that the US military must do its part by being ready for war.
Without forecasting a conflict, Mattis emphasized that diplomacy stands the best chance of preventing a war if America's words are backed up by strong and prepared armed forces.
"My fine young soldiers, the only way our diplomats can speak with authority and be believed is if you're ready to go," Mattis told several dozen soldiers and airmen at the 82nd Airborne Division's Hall of Heroes, his last stop on a two-day pre-holiday tour of bases to greet troops.
Secretary of Defense Jim Mattis enters Michie Stadium before the 2017 graduation ceremony at West Point. Army photo by Michelle Eberhart.
The stop came a day after Mattis visited the American Navy base at Guantanamo Bay, Cuba, becoming the first defense secretary to visit in almost 16 years.
Mattis' comments came as the UN Security Council unanimously approved tough new sanctions against North Korea, compelling nations to sharply reduce their sales of oil to the reclusive country and send home all North Korean expatriate workers within two years. Such workers are seen as a key source of revenue for North Korean leader Kim Jong Un's cash-strapped government.
President Donald Trump and other top US officials have made repeated threats about US military action. Some officials have described the messaging as twofold in purpose: to pressure North Korea to enter into negotiations on getting rid of its nuclear arsenal, and to motivate key regional powers China and Russia to put more pressure on Pyongyang so a war is averted.
The Daily Telegraph reported earlier this week that the Trump administration had had "dramatically" stepped up preparations for a "bloody nose" attack to send Pyongyang a message.
Also this week, when asked about the US's stance toward the stand-off with North Korea, National Security Adviser H.R. McMaster, said the US had "to be prepared if necessary to compel the denuclearization of North Korea without the cooperation of that regime."
For the military, the focus has been on ensuring soldiers are ready should the call come.
At Fort Bragg, Mattis recommended the troops read T.R. Fehrenbach's military classic "This Kind of War: A Study in Unpreparedness," first published in 1963, a decade after the Korean War ended.
"Knowing what went wrong the last time around is as important as knowing your own testing, so that you're forewarned — you know what I'm driving at here," he said as soldiers listened in silence. "So you gotta be ready."
The US has nearly 28,000 troops permanently stationed in South Korea, but if war came, many thousands more would be needed for a wide range of missions, including ground combat.
The retired Marine Corps general fielded questions on many topics in his meetings with troops at Guantanamo Bay naval base in Cuba and Naval Station Mayport in Florida on Thursday and at Camp Lejeune and Fort Bragg in North Carolina on Friday. North Korea seemed uppermost on troops' minds as they and their families wonder whether war looms.
Asked about recent reports that families of US service members in South Korea might be evacuated, Mattis stressed his belief that diplomacy could still avert a crisis. He said there is no plan now for an evacuation.
"I don't think it's at that point yet," he said, adding that an evacuation of American civilians would hurt the South Korean economy. He said there is a contingency plan that would get US service members' families out "on very short notice."
Mattis said he sees little chance of Kim disrupting the Winter Olympics, which begin in South Korea in February.
"I don't think Kim is stupid enough to take on the whole world by killing their athletes," he said.
Mattis repeatedly stressed that there is still time to work out a peaceful solution. At one point he said diplomacy is "going positively." But he also seemed determined to steel US troops against what could be a costly war on the Korean Peninsula.
Defense Secretary Jim Mattis and South Korean Minister of Defense Song Young-moo visit the Demilitarized Zone between North and South Korea during a visit to the Joint Security Area in South Korea, Oct. 27, 2017. DoD photo by US Army Sgt. Amber I. Smith.
"There is very little reason for optimism," he said.
Mattis is not the only senior military official cautioning troops to be ready for conflict.
Marine Corps Commandant Gen. Robert Neller told Marines in Norway this week that he saw a "big-ass fight" in the future, cautioning them to remain alert and ready. Neller said he believed the Corps' focus would soon shift away from operations in the Middle East toward "the Pacific and Russia."
"I hope I'm wrong, but there's a war coming," Neller told Marines in Norway according to Military.com.