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

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.