Speaking to reporters at his golf club in Bedminster, New Jersey, where the President is currently on a “working vacation,” Trump announced Tuesday that he is unafraid to take action against a nuclear-armed North Korea.

In a remark described by one of his top aides as “unplanned and spontaneous,” Trump drew an ill-defined red line with North Korea, vowing, “North Korea best not make any more threats to the United States.” He went on to issue a grave warning that “They will be met with fire and fury like the world has never seen.”

Trump then directed his ire at North Korea’s unstable leader, Kim Jong-un, saying, “He has been very threatening beyond a normal state, and as I said, they will be met with fire and fury, and frankly power the likes of which this world has never seen before.”

In language that The New York Times characterized as “chilling” and “evok[ing] the horror of a nuclear exchange,” the commander-in-chief of the world’s largest military broke with decades of diplomatic norms by issuing the warning.

Historians agreed that Trump’s remark was without precedent, and an article in Time Magazine compared the statement to President Truman’s speech announcing that the U.S. military had dropped a nuclear bomb on Hiroshima in 1945.

The day after Trump made his statement marked the 72nd anniversary of the U.S. launching the second and final nuclear attack of WWII, which killed tens of thousands instantly in the Japanese city of Nagasaki.

North Korea wasted no time responding to Trump’s fiery rhetoric in kind. A statement issued by the Strategic Force of the Korean People’s Army threatened a strike that would lead to “an enveloping fire” on Guam, a strategic military base that has been a U.S. territory since 1898.

Guam is home to Navy, Air Force, and Coast Guard bases and is protected by a Terminal High Altitude Area Defense (or THAAD) system designed to shoot down ballistic missiles. It is the closest U.S. territory to North Korea.

North Korea’s General Kim Rak Gyom also issued a statement, saying, “Sound dialogue is not possible with such a guy bereft of reason and only absolute force can work on him.”

The statement went on to detail North Korean plans to launch four missiles into the ocean along Guam’s coast as a show of force, saying that once prepared, the plan would move forward upon orders from commander-in-chief Kim Jong-un.

On July 4, North Korea successfully tested an intercontinental ballistic missile that experts say could deliver a miniature nuclear warhead to major U.S. cities for the first time.

The U.S. intelligence community has recently come to the consensus that North Korea does, in fact, have the technology to sufficiently miniaturize a warhead and attach it to an ICBM.