SEOUL — South Korea opened the second act of its agonizing leadership crisis with the ouster of Yoon Suk-yeol as president on the day after President Trump imposed devastating tariffs that are sure to fuel the presidential ambitions of the opposition leader primarily responsible for Mr. Yoon’s downfall four months after his ill-conceived attempt at bringing Free Korea’s fledgling democracy under martial law.

The leader of the Democratic Party, or Minju, Lee Jae-myung,  praised the Constitutional Court for its “wise decision” upholding the impeachment motion that he had engineered in the national assembly in retaliation for Mr. Yoon’s martial law decree of December 3. He rescinded his decree six hours after issuing it — and three hours after the assembly voted it down. The assembly, led by Mr. Lee’s Minju party, impeached Mr. Yoon 11 days later, stripping him of his power but not his title, which he lost with the court approving his impeachment after often contentious hearings.

Mr. Yoon’s precipitous fall from grace ends his presidency more than three years after he was elected president over Mr. Lee by less than one percentage point, promising to restore close military ties with South Korea’s American ally and pursue tough defenses against North Korea.

As president, Mr. Yoon enthusiastically endorsed joint Korean-American military exercises, which his left-leaning predecessor, Moon Jae-in, had opposed, and forged much improved relations with Japan, as seen in the trilateral summit that President Biden hosted at Camp David in August 2023 with Mr. Yoon and Japan’s then prime minister, Fumio Kishida.

Mr. Lee, facing multiple criminal charges of corruption in real estate and political dealings in his days as governor of the province surrounding Seoul and mayor of a city just south of the capital, remains the front-runner in the snap election that must be held within 60 days of Mr. Yoon’s ouster. He is likely to maintain the alliance with Washington while looking for dialogue with North Korea’s leader, Kim Jong-un, just as President Trump is doing.

The tariffs imposed by Mr. Trump should make it far easier for Korean leftists to cool down the warm ties that Mr. Yoon had formed with Washington. The foreign minister, Cho Tae-yul, in a meeting in Brussels with Secretary of State Rubio and Japan’s foreign minister, Takeshi Imaya, asked Washington to reconsider raising tariffs considering “the implications for its allies, the importance of close South Korea-U.S.-Japan security cooperation, economic collaboration and South Korea’s track record of investment in the U.S.,” according to the foreign ministry in Seoul.

The views of Mr. Cho, as a member of the conservative government that Mr. Yoon put into place, were likely to be far more measured than those of proteges of Mr. Lee and others in the Minju, many of whom were jailed in their youths for opposing quasi-dictatorial regimes 

The government for now remains in the hands of the acting president, Han Duck-soo, a cautious bureaucrat with a long career in economic affairs. He had served as prime minister under Mr. Yoon and Korea’s ambassador at Washington.

Mr. Han promised to do “my utmost to oversee a smooth and fair presidential election” and asked bureaucrats to remain “neutral” politically. TV channels repeatedly showed the acting chief justice, Moon Hyung-tae, declaring, “The constitutional order and the repercussions from the defendant’s violations of the law are grave, making the benefits of protecting the Constitution by dismissing the defendant overwhelmingly larger than the national losses from dismissing the president.”

Thousands of policemen lined nearby streets, blocking protesters from getting near the court. Anti-Yoon demonstrators burst into cheers, waving signs in Korean saying “Victory” and “Democracy Wins.” On an avenue near Mr. Yoon’s presidential residence, rightists, waving American and Korean flags, burst into shrieks of grief, resolving to “fight for democracy” and “oppose the unjust decision of the traitorous court.” 

Mr. Yoon did not appear on TV but had his lawyers relay his apology for “not being able to live up to your hopes and expectations.” It was, he said, “the greatest honor of my life to serve our nation” and he was “sincerely grateful for your unwavering support and encouragement.” The crowd, of course, cheered and wept as these words were read to them.

For Mr. Yoon, the ordeal is far from over. He and a number of his allies still face the charge of “insurrection” for staging his abortive coup.