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Editor’s Note: YJ Fischer served at the State Department from 2012-2016. She co-wrote the 2016 Democratic Party platform and served on the Clinton-Kaine transition team. The views expressed in this commentary are her own.

(CNN) —  

While President Donald Trump is walking away from the nuclear agreement with Iran, he is doubling down on negotiating with North Korea. Even as he scorns “a horrible, one-sided deal that should have never, ever been made,” he appears prepared to offer North Korea a concession that would be far more damaging than anything President Barack Obama even contemplated offering Iran: removing the roughly 28,500 US troops stationed in South Korea.

Such a move would be a huge win for North Korea and China and a major danger for South Korea and Japan, and it would send the message to other rogue regimes that it pays to develop nuclear weapons. If that’s the Trump Doctrine, it would be a disaster for the United States and the world.

YJ Fischer
Nick Merrill
YJ Fischer

Imagine if during the negotiations with Iran, Obama had offered to pull US troops out of the Middle East or to stop defending Israel? That wouldn’t just have been a non-starter – Republicans in Congress probably would have started impeachment proceedings.

NATO’s first Secretary General Lord Ismay famously said that the Atlantic alliance was designed to “keep the Soviet Union out, the Americans in and the Germans down.” Similarly, Trump would do well to think of US troops in South Korea as keeping the Chinese out, the Americans in and the North Koreans down.

But Trump doesn’t seem to appreciate the national security benefits of our alliances or the risks of weakening them. Instead, he appears to view alliances as a burden. During the 2016 presidential campaign, Trump suggested that South Korea and Japan take care of defending themselves, even if they have to develop nuclear weapons to do so.

Trump either doesn’t know or doesn’t care that bringing US troops home would actually be more expensive than keeping them where they are, because South Korea pays nearly half the cost of their upkeep and is funding 92% of their new base. According to by NBC News, Trump wanted to pull all troops out of South Korea in February, but was talked down by Chief of Staff John Kelly. And this month, according to the New York Times (citing several people briefed on the deliberations), Trump ordered the Pentagon to develop options for drawing down troops. (The White House denies this.)

If Trump is ready to trade away America’s military presence in South Korea even before sitting down with North Korean leader Kim Jong Un, he may offer the concession regardless of what the dictator puts on the table in return.

But that would be a grave mistake for the following reasons:

First, it would significantly increase North Korea’s ability to threaten South Korea’s security. North Korea has long wanted to reunify the Korean peninsula under its control. Admiral Harry Harris, the top US commander in the Pacific who has been nominated to be ambassador to South Korea, told Congress that this is Kim’s primary goal.

Even without its nuclear arsenal, North Korea has thousands of artillery cannons and rocket launchers in striking distance of Seoul and, analysts says, could fire more than 300,000 rounds within the first hour of a conflict. But while North Korea has periodically engaged in military provocations with the South, it hasn’t launched a full-scale invasion since 1950 because American troops are too great a deterrent. If US forces were to withdraw, Kim could attempt to overwhelm South Korea in a quick attack – and he probably wouldn’t even have to use nuclear weapons to do it.

Second, withdrawing American troops from South Korea would be a significant win for China with implications well beyond the Korean Peninsula. China views all of East Asia as its sphere of influence and wants a free hand to pursue its territorial ambitions in the South and East China Seas. According to the Cipher Brief, a digital, security-based platform, China’s military has been enhancing its array of anti-access and area-denial capabilities, such as anti-ship ballistic missiles, to make it harder for American forces to operate in the region. If Trump were to pull American troops out of South Korea, it would remove a key counter-balance to China’s growing power and increase Beijing’s leverage.

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Finally, making such a major concession would establish a dangerous precedent. Even under the best-case scenario, where negotiations with North Korea lead to a credible agreement to dismantle its nuclear weapons, Trump needs to be careful about the benefits he is willing to give Kim in return. It’s like negotiating with terrorists or hostage-takers. The US should not signal to other rogue regimes that nuclear blackmail is the best way to extract major concessions from us. That would likely lead to an arms race, with more countries seeking to follow North Korea’s example.

So far, the administration has offered little clarity about what Trump is willing to trade away in exchange for a deal on North Korea’s nuclear program. Withdrawing US troops from South Korea shouldn’t be on the list.