Cha also objected to the administration's threats to tear up a bilateral trade deal with Seoul that President Trump has called unfair to American companies. Last week, the administration imposed new tariffs on imports of washing machines and solar energy panels, a move criticized by the South Korean government.
The White House had spent months conducting a security and financial background check on Cha, and U.S. officials formally notified Seoul in December of Trump's intent to send his nomination to the Senate. South Korean officials quickly signed off on Cha, a formal process in international affairs known as "Agrément."
Newspapers in Seoul hailed his pending appointment in December, and South Korean diplomats in Washington were pressing the White House to formally nominate him to the Senate, hoping to have him in place in time for the Winter Olympics, which start Feb. 9 in PyeongChang.
But the nomination never came. A senior administration official confirmed this week that the White House had moved on to other potential candidates.
"We have yet to nominate anyone for the post, but it is our intention to do so as soon as we can find the appropriate candidate," said the official, who spoke on the condition of anonymity to discuss a personnel matter.
Trump addressed the threat posed by North Korea in his State of the Union address Tuesday night, calling its leadership "depraved" and threatening, but he did not lay out any new policies toward Pyongyang.
A person familiar with the nomination process said a red flag was raised during Cha's background check that led officials to determine he could not serve in the ambassador's post. But this person, who was not authorized to speak publicly, did not offer specifics.
Officials at the White House and State Department did not respond to several requests for comment on Cha's situation. Cha declined to comment for this story, but in an opinion piece published in The Washington Post on Tuesday evening, he referred to his candidacy for the South Korea post being in the past and warned against the "bloody nose" strategy.
The "answer is not, as some Trump administration officials have suggested, a preventive military strike," Cha wrote. "Instead, there is a forceful military option available that can address the threat without escalating into a war that would likely kill tens, if not hundreds, of thousands of Americans."
The abrupt derailment of Cha's nomination comes after months of growing tensions between the United States and North Korea. The Trump administration and the United Nations have enacted stiff economic sanctions and have sought to increase diplomatic pressure on Pyongyang over its continued nuclear and ballistic missile tests.
Trump has threatened to use military force if necessary and engaged in a war of words with North Korean dictator Kim Jong Un. Last month, Trump wrote on Twitter that his "Nuclear Button" is "much bigger & more powerful."
Though Washington and Seoul agreed to temporarily pause joint military exercises through the two-week Olympics and the North agreed to send a delegation to the Games, experts said the tensions are likely to continue to escalate.
Marc Knapper, a civil servant who is the charge d'affaires at the U.S. Embassy in Seoul, has overseen diplomatic operations there since former ambassador Mark Lippert, appointed by President Obama, left shortly before Trump took office last year.
Cha is well known in Washington's foreign policy circles, having served as Asia director at the NSC under Bush. He participated in the Six-Party talks with North Korea as the deputy head of the U.S. delegation and was generally considered hawkish on security issues involving the North.
He currently serves as the Korea program chair at the Center for Strategic and International Studies and is director of Asian studies at Georgetown University's School of Foreign Service. Cha did not sign either of the two "never Trump" letters circulated during the campaign by national security establishment figures — who have mostly been blackballed for administration jobs since Trump won the election.
Experts said the White House's presidential personnel office handles vetting of political appointees. Cha's background check was said to be extensive, and he signed an ethics letter agreeing to step down from CSIS and Georgetown once he was confirmed by the Senate.
A former Obama administration official who knows Cha but was unfamiliar with the specifics of his case said that a background vetting issue raised this late in the process — after Agrément was completed — was highly unusual.
"The South Korean government received Agrément and granted a quick turnaround before the end of [last] year," said the former Obama official, who spoke on the condition of anonymity to provide information on the vetting process.
"It's inconceivable that there would be anything so complex in the portfolio of an academic that wouldn't be quickly resolved" months ago, this person said.
Two people who know Cha suggested Trump aides had second thoughts on his nomination over policy disagreements with him concerning how to respond to North Korea's nuclear and ballistic missile testing. Cha had forwarded articles to the NSC about the risks of a preemptive U.S. strike, and his colleagues at CSIS, including another former Bush administration official, Michael Green, had published their own analyses warning against such a strategy.
During a forum Cha hosted at CSIS in September, former U.S. secretary of state Madeleine Albright warned that the Trump administration should not "ramp up the rhetoric" against North Korea.
Sitting next to Cha, Albright emphasized the merits of diplomacy and said: "It would be great if we had an ambassador in Seoul" — prompting laughs from the audience.
Vice President Pence will lead the official U.S. delegation for the opening ceremonies at the Olympics.