Defense Secretary Jim Mattis advised U.S. troops on Friday that the best way to prevent war is to be “ready to go.”

“My fine young soldiers, the only way our diplomats can speak with authority and be believed is if you’re ready to go,” Mattis reportedly said to soldiers at the 82nd Airborne Division’s Hall of Heroes in North Carolina.

According to the Associated Press, Mattis warned that “storm clouds were gathering” over the Korean Peninsula.

On Friday, the U.N. Security Council unanimously approved new sanctions on North Korea in response to the latest launch of a ballistic missile.

President Trump has repeatedly warned that the U.S. could take action against Pyongyang if it continues to fire ballistic missiles and threaten allies in the region.

According to the AP, Mattis recommended that the troops read T.R. Fehrenbach’s military classic, This Kind of War: A Study in Unpreparedness, first published in 1963.

“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,” Mattis explained. “So, you gotta be ready.”

There are currently 28,000 U.S. troops permanently stationed in South Korea.

Mattis stressed that though diplomacy is “going positively” in the Korean region, “[t]here is very little reason for optimism.”

While delivering a keynote address at the Association of the U.S. Army’s annual meeting in October, Mattis told troops they “must stand ready” as provocations by North Korean leader Kim Jong Un continue.