Two US congressmen have made public a letter from the US Joint Chiefs of Staff which says that an American ground invasion of North Korea is the only way to completely eliminate its nuclear arsenal. The assessment undercuts impressions that an attack by US missiles and warplanes alone would be adequate to neutralize such a threat.

A copy of the letter, which was obtained by Newsweek, was sent on October 27 to representatives Ted Lieu and Ruben Gallego of California on behalf of Defense Secretary James Mattis. It was a response to a letter the two Democrats had sent to Mattis in late September expressing their concerns about the possibility of war with North Korea and asking the Pentagon to provide a detailed report on the potential consequences of such a conflict.

Mattis reportedly issued a response via the Office of the Chairman of the Joint Chiefs of Staff. The relevant paragraph of the letter said, “The only way to ‘locate and destroy — with complete certainty — all components of North Korea’s nuclear weapons programs’ is through a ground invasion.” The response was written by US Navy by Rear Adm. Mike Dumont.

“One of the problems that we have with the North Korea situation is that we have very little intelligence on their regime,” Lieu told Newsweek. This makes it difficult to pinpoint where their nuclear weapons and other military assets are, and why a ground invasion would be needed to destroy them.

Dumont also noted a possibility that the North might unleash biological weapons “as an option” in the event of war.

An article published in early October by 38 North, a respected specialist website on North Korea, estimated that over 2 million lives could be lost should North Korea attack Seoul and Tokyo with its nuclear arsenal.