Demonstrators hold placards and wave flags as they march toward the presidential Blue House, after the Constitutional Court upheld a parliamentary vote to impeach President Park Geun-hye for corruption. (Seongjoon Cho/Bloomberg News)

THE REMOVAL of President Park Geun-hye must be seen as a testament to South Korea’s young democracy. For all the disruption and difficulty, the nation has managed to perform one of the most tricky maneuvers in a democratic system: undertaking the transfer of power under the rule of law at a time of extreme duress. The agility to make the hard decisions and hand off the baton without a bloody coup is a sign of strength that distinguishes democracy from dictatorship. Much credit goes to the spirit of nonviolent protest that filled the months-long street demonstrations.

At the same time, the whole affair further exposed the seamy collusion between the chaebol conglomerates and political power. Ms. Park was removed from office when the Constitutional Court upheld a parliamentary vote to impeach her for corruption. The Post’s Anna Fifield reports that the court found she had helped her friend Choi Soon-sil extract bribes from South Korean conglomerates, including Samsung; personally asked big business for donations; leaked confidential documents to Choi; tried to cover up her wrongdoing; and lied about it. South Korea must clean up this corruption to reestablish trust in government.

But much sooner, the country faces serious external challenges. Ms. Park took an unflinching stand toward Kim Jong Un’s dangerous and unpredictable regime in North Korea. The arrival last week in South Korea of the first wave of equipment to install a U.S. missile defense system known as Terminal High Altitude Area Defense, or THAAD, and ongoing “Foal Eagle” joint military exercises with the United States are products of this determined and sound approach. Now, Seoul faces a moment of vulnerability and must not be intimidated. The North has attempted to crudely exploit the political turmoil, including the launch last week of a volley of four missiles toward Japan.

China has also behaved thuggishly, encouraging boycotts of South Korean businesses to protest the missile defense deployment. Beijing’s latest diplomatic proposal, that North Korea freeze its nuclear and missile programs in exchange for a U.S. halt to joint exercises with the South, was a risible nonstarter that once again called into question whether China can ever be a serious brake on the reckless and dangerous Mr. Kim.

In the upcoming South Korean elections, polls show a leading candidate is Moon Jae-in, a progressive proponent of the “sunshine policy” of engagement with North Korea. The strategy is meant to ease tension and open up the closed state, but this hardly seems like the right time for it. Secretary of State Rex Tillerson, who travels to Japan, South Korea and China this week, should use his trip to drive home to South Korea the wisdom of standing firm against pressure from the North and from China, while strongly reassuring Seoul of the U.S. commitment to its security. Mr. Tillerson has made his task that much harder with a foolish decision to exclude the media from his traveling party, breaking with decades of precedent. This is no time for muffled voices.