Ash Carter: A US pre-emptive strike on North Korea could lead to 'invasion' of South Korea

Former Defense Secretary Carter also said China fears a unified Korea.

“Even though the outcome is certain, it is a very destructive war,” the former defense secretary said, noting that the South Korean capital of Seoul, with about 25 million area residents, is on the border between the two countries. “So one needs to proceed very carefully here.”

But Carter said the Chinese have been “unwilling” to take on that role because, in the short term, they are “fearful of a collapse of North Korea or a war” that would result in a “unified Korea allied with the United States on their border.”

"In strategic affairs, clarity and consistency are very important,” Carter said. “This is our government and our foreign policy, and we all need to wish it success. But there does need to be clarity and consistency."