President Trump can't just start a second Korean War by himself — that is, unless he wants to abrogate a six-decades-old treaty, scholars and the president of South Korea say.

The president's vow to meet North Korea's threats with "fire and fury" prompted South Korean president Moon Jae-in last week to reassure his people that the American president is bound by treaty not to start a war on the Korean Peninsula without South Korea's approval.

"I am saying any military action to be taken on the Korean Peninsula requires South Korea's consent unless it is taken outside the peninsula," Moon said Thursday, in a nationwide televised news conference, in which he also made clear he would not approve of a preemptive strike against North Korea at this time.

"This is a firm agreement between South Korea and the United States. The people can be assured that there will be no war," Moon said.

Moon appeared to be referring to the mutual defense treaty between the U.S. and the Republic of Korea signed in October of 1953, at the end of the Korean War.

Article 2 of the treaty says "The Parties will consult together whenever … either of the Parties is threatened by external armed attack," and "will take suitable measures in consultation and agreement."

The treaty does not rule out unilateral action if the United States is attacked or about to be attacked by North Korea, but Moon said the U.S. is still obligated to first consult with the South, which would bear the brunt of any North Korean retaliation.

"Basically, the Republic of Korea is the one directly and mostly affected by North Korea's nuclear and missile issues," Moon said, insisting the U.S. must not act without Seoul's permission. "I am confident it will sufficiently consult with South Korea in advance if such action may increase tension between the South and the North."

The 1953 treaty is a relatively short document and does not spell out all the various contingencies that could arise. But over the years, the idea that the U.S. would not initiate hostilities with the North over the objections of the South have become firmly established, according to scholars.

"The question of unilateral military action is an important one in this context," said Nicholas Eberstadt, a Korea expert at American Enterprise Institute. "We have a U.S.-ROK military alliance in which under existing protocols and doctrine military action has to be approved by both presidents."

And with 90 percent of South Korea's population within artillery range of the North's thousands of big guns and rocket launchers, any South Korean leader is going to be extremely hesitant to unleash the horrors of war.

"Think about what would be required for a South Korean president, sitting in Seoul to say, ‘Go ahead, Donald, Blast them,'" Eberstadt said. "It's not something that would be undertaken lightly."

Still, if Trump authorized a preemptive strike against the North in the name of protecting the U.S., without the approval of Seoul, there would be little to stop him.

That scenario could play out if, for instance, U.S. intelligence determined Kim Jong Un was about to launch a nuclear-tipped missile at a major American city.

"If the U.S. president did move unilaterally in a circumstance like that, against the [wishes] of a South Korean president, the U.S.-ROK defense alliance would be over in a heartbeat," Eberstadt said.

Sen. Lindsey Graham, R-S.C., has said Trump has told him he is willing to go to war to prevent North Korea from having the ability to hit the U.S. with a nuclear weapon.

"There is a military option: To destroy North Korea's nuclear program and North Korea itself," Graham said this month.

"You're making our president pick between regional stability and homeland security," he said. "There will be a war with North Korea over their missile program if they continue trying to hit America with an ICBM. He's told me that, and I believe him."

Graham argued that as horrible as a second Korean War would be, Trump believes "if thousands die, they're going to die over there. They're not going to die over here."

Still, all of Trump's advisers are telling him all-out war on the Korean Peninsula would be a catastrophe on a scale not seen since World War II.

"Under a sufficiently dire crisis, that might be a cost that an American president would be willing to accept," Eberstadt said. "But that's not something that any American decision-maker is going to take lightly."