The situation also has consequences beyond domestic politics.
Up until last month, the Biden administration had sung Yoon’s praises, delighted by his willingness to work with Washington to tackle the security threats posed by North Korea and China. The US put a lot of effort into helping South Korea repair its strained relations with Japan, so the three countries could address these issues together.
Mr Blinken’s ongoing visit to Seoul, where he will meet South Korean foreign minister Cho Tae-yul on Monday, therefore comes at a difficult time for these two allies.
Yoon did not tell the US about his plans to impose martial law, meaning Washington did not have the chance to dissuade him and was unprepared for the chaos that ensued.
Blinken will not want to be drawn on the current political situation. He will instead want to focus on preserving the trilateral cooperation between Seoul, Washington and Tokyo beyond Biden’s tenure.
Speaking during a joint presser on Monday, Blinken said the US had “full confidence” in South Korea’s institutions, and reaffirmed the US government’s “unwavering support for the Korean people as they work tirelessly to uphold those institutions”.
“Over the past four decades Korea has written one of the most powerful, inspiring democratic stories in the world,” Blinken said.
Korea’s democracy has been tested in recent weeks – just as American democracy has faced challenges throughout our history. But you are responding by demonstrating your democratic resilience.”
But it’s hard to disentangle the domestic and geopolitical situations. South Korea could be months away from electing a new president, and that leader may well want to break with Yoon’s foreign policies.
Trump, who enters the White House in a fortnight, will also pursue his own agenda.
Additional reporting by Hosu Lee and Leehyun Choi in Seoul