"The U.S. would oust the communist regime in North Korea if it uses its nuclear weapons or launches an all-out invasion on South Korea and the 28,500 American troops stationed there, national security sources say.
The Obama administration has not articulated such a far-reaching retaliation, even as North Korean leader Kim Jong-un threatens to attack both the South Korean and the U.S. mainland.
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But national security sources say it is a common assumption within the Pentagon and U.S. Pacific Command that a full-force attack by Pyongyang would put in place a contingency plan of massive retaliation against the North aimed at bringing down the Stalinist regime. President Obama then would be faced with making the war decision.
"I have been told by a senior general that an attack by the North means regime change," a senior congressional defense aide told The Washington Times. "I was told the same thing when I visited Pacific Command [based in Hawaii]."
The South Korean government would not allow the U.S. to launch a preemptive regime change of the North, as the George W. Bush administration executed in Iraq in 2003.
However, an all-out war by the North would change the reasoning.
The Bush administration was more open about its retaliation plans. Then-Vice President Richard Cheney said an invasion of the South or the use of nuclear weapons would mean the end of the Kim dynasty.
The Pentagon, with new strategic guidance from Mr. Obama in hand, has been updating its war plans for all regions, including the Korean Peninsula.

A source familiar with the thinking has told The Times that the plans include scenarios for the regime imploding on its own and an allied invasion in response to North Korea launching a war on the South.
The plan calls for the South Korean government and its armed forces to take the lead in post-war stability operations to transform the North and achieve unification of the Koreas.
As part of a show of force, last week the Pentagon dispatched B-2 stealth bombers from Whiteman Air Force Base in Missouri to take part in joint exercises with the South Koreans.
The B-2 is an overt offensive weapon, and its use in a war would be to hit targets held dear by the North, such as headquarters, presidential sites and nuclear facilities.
"We would respond with massive airpower and destroy their air force and air defenses within 50 hours, which means we could destroy their ground forces, with South Korean ground forces, in less than 60 days," said retired Air ForceLt. Gen. Thomas McInerney. "Yes, there would be carnage, but North Korea would be defeated decisively and quickly."
Gen. McInerney, who did war planning on the Korean Peninsula in the 1980s, said it is China's responsibility to keep Mr. Kim from acting on his threats to attack the United States and invade South Korea.
"Don't worry about China. If they let it happen, it's their problem," he said. "They are not going to war. China is not going to throw in a hundred thousand troops. China has got to keep this guy on a short leash. China is making so much money. They don't want ill will around the world, trade embargoes. It's not in their interest."
Asked Tuesday if the Pentagon has a plan to preemptively strike the North, George Little, spokesman for Defense Secretary Chuck Hagel, said: "Let me be very clear that the United States position is, we want peace and stability on the Korean Peninsula. For over 60 years, we've had an alliance with South Korea, and top priority of that alliance is to ensure peace and stability on the peninsula and in the region."
Mr. Little added: "We do have options at our disposal to respond effectively to any North Korean provocation. We have plans in place with our South Korean allies, and naturally we hope never have to put any of these plans into place. The goal — let me reiterate — is to protect peace and stability on the peninsula."
Secretary of State John F. Kerry said Tuesday: "The United States will do what is necessary to defend ourselves and defend our allies, Korea and Japan. We are fully prepared and capable of doing so. And I think [North Korea] understands that."


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