Trump's initial pick for ambassador to S. Korea, Victor Cha, was a hawk on North Korea... but he felt Trump's bellicose approach put American lives, and South Korean lives at risk... so he is out.
Trump has mused about the possibility of a 9-11 scale event helping his popularity

https://www.washingtonpost.com/politics/disagreement-on-north-korea-policy-could-derail-white-house-choice-for-ambassador-to-south-korea/2018/01/30/3a21191c-05da-11e8-94e8-e8b8600ade23_story.html?utm_term=.792c28d8eed8

The White House’s original choice for U.S. ambassador to South Korea is no longer expected to be nominated after he privately expressed disagreement in late December with the Trump administration’s North Korea policy, according to people familiar with the matter.

Victor D. Cha, an academic who served in the George W. Bush administration, raised his concerns with National Security Council officials over their consideration of a limited strike on the North aimed at sending a message without sparking a wider war — a risky concept known as a “bloody nose” strategy.