Elbridge Colby, the deputy assistant secretary of defense for strategy and force development during the Donald Trump administration, speaks at his office in Washington, May 6. Yonhap

South Korea and the United States need to revise their combined wartime contingency plans if they involve America's dispatch of massive military assets to fend off a North Korean invasion, a former Pentagon official has said, stressing the U.S.' imperative to focus on the "decisive" threat from China.

In an interview with Yonhap News Agency Monday, Elbridge Colby, the deputy assistant secretary of defense for strategy and force development during the Trump administration, also expressed his support for the swift transfer of wartime operational control (OPCON) from Washington to Seoul, saying the Asian ally should undertake "overwhelming" responsibility for its own defense.

Laying out his vision for the role of the 28,500-strong U.S. Forces Korea (USFK), he called for an overhaul to make it "more relevant" to handling China-related contingencies rather than being held "hostage" to addressing North Korean threats.

Colby shared his thoughts on America's security and foreign policy, emphasizing he does not speak for former President Donald Trump or his campaign. He has been mentioned as a potential candidate for national security advisor should Trump return to the White House.

"I think we need to have a plan that is based on reality. If you are assuming that the United States is going to break its spear, if you will, fighting North Korea, that is an imprudent assumption for us to make or for you to make," he said.

"To the extent that we are currently planning on sending massive amounts of forces to Korea that would decrement from our ability to deal with the Chinese, I think we need to revise that. I think we need to have a plan for the defense of South Korea that the U.S. and the president of the U.S. could rationally implement," he added.

Seoul and Washington have their operational plans in place to prepare for contingencies on the Korean Peninsula. Those plans are known to be based on the premise that the U.S. will deploy a large number of augmented forces and other military assets in support of South Korea.

Colby asserted that if the U.S. gets heavily involved in a war with North Korea, it would be a "perfect distraction" when America should prioritize countering what he called the "hardest" threats from China.

"South Korea is going to have to take primary, essentially overwhelming responsibility for its own self-defense against North Korea because we don't have a military that can fight North Korea and then be ready to fight China," he said.

Elbridge Colby, the deputy assistant secretary of defense for strategy and force development during the Donald Trump administration, speaks at his office in Washington, May 6. Yonhap

The prominent security expert took note of an "asymmetry of perspective" between Seoul and Washington when it comes to threat perception.

"The fundamental fact is that North Korea is not a primary threat to the U.S. It would not be rational to lose multiple American cities to just deal with North Korea. That's a different calculation for South Korea," he said. "We need to realistically evaluate an approach."

Colby laid out his idea of contingency operations in which America's intervention occurs when China gets directly involved in a conflict on the peninsula.

"That is what we should be asking (South Korea). Hold your own as much as possible, and if the Chinese get directly involved, that's when the Americans would come in," he said.

"China is the decisive threat. So we need to retain our capacity to make sure that if (you are) the heavyweight boxing champion ... you don't want to get in a middle-weight fight ... (as) you are so bruised and tired that you lose the heavyweight fight (with China)."

On the Taiwan contingency, he said that the U.S. should not ask Seoul to get directly involved for the defense of the island democracy considering persistent threats from Pyongyang.

Colby underscored the need for a USFK overhaul to confront Chinese threats — a suggestion that would mark a shift away from its current focus on defense against North Korean threats.

"U.S. forces on the peninsula in my view should not be held hostage to dealing with the North Korean problem because that is not the primary issue for the U.S.," he said. "The U.S. should be focused on China and the defense of South Korea from China over time."

He added that U.S. forces that are meant to defend South Korea against a Chinese attack should not be in Korea.

"Because if they were in Korea, they would be subject to massive preemptive attack at short range," he said. "South Korea is so close to China that it's under massive Chinese missile threat not to mention North Korea."

In the face of evolving North Korean threats, Colby said "all options" should be on the table to ensure South Korea's security, including its nuclear armament.

Elbridge Colby, right, the deputy assistant secretary of defense for strategy and force development during the Donald Trump administration, speaks at his office in Washington, May 6. Yonhap

Asked what if Seoul faces costly economic sanctions in case of its nuclear option, he said that Washington should not sanction Seoul when the security measure was crafted through mutual coordination.

"It would be self-defeating and foolish for us to simultaneously not provide South Korea with a viable defense umbrella and then threaten to sanction it when it decides with us to take measures that provide for security in the face of a tremendous nuclear buildup b North Korea and China," he said.

He threw his weight behind the expeditious OPCON handover from the U.S. to South Korea.

"I would say I agree with the OPCON transfer. I think the more (South Korean forces) can operate autonomously and independently, the better," he said.

He went on to say, "The idea of South Korea assuming primary responsibility should happen as soon as possible, like now."

South Korea handed over operational control over its troops to the U.S.-led U.N. Command during the 1950-53 Korean War. It was then transferred to South Korea-U.S. Combined Forces Command when the command was launched in 1978.

Seoul and Washington have been working on meeting a wide range of conditions needed for OPCON transition. Conditions include South Korea's capabilities to lead combined forces, its strike and air defense capabilities as well as a regional security environment conducive to such a handover.

Portraying the conditions as "not inherent," Colby highlighted complex regional security conditions that call for Seoul to provide for its own security.

On nuclear diplomacy with North Korea, he said that the idea of North Korea's complete denuclearization appears "impossibly far-fetched." He highlighted the need to focus on something "attainable."

"I think in terms of our policy goal, what we should be thinking about ... it's got to be more like arms control, particularly focused on limiting the range of North Korean ICBMs," he said, referring to intercontinental ballistic missiles.

Colby is currently principal and co-founder of the Marathon Initiative, a Washington-based research organization. He served as deputy assistant secretary of defense from 2017-2018. He is a graduate of Harvard College and Yale Law School. (Yonhap)