The imminence of President-elect Trump’s return to the White House has foreign policy critics warning of what they see as disasters from Ukraine to northeast Asia.

If America under Trump cannot stop the Russians in Europe, the former and future president’s one-time national security adviser, John Bolton, asked, “What can we do for Taiwan?”

Mr. Bolton, in a zoom conversation staged by the Institute for Corean-American Studies, said he “would be in favor if South Korea decided they wanted some of their troops to get experience in Ukraine,” just as the North Koreans forces are doing on Russia’s side.

“Our declared position is that Ukraine should get full restoration, territorial integrity,” he said. ”We’ve allowed the current situation to persist for three years.” If Trump “follows the pattern of his first term, with respect to alliances like NATO or bilateral alliances” with the likes of Japan and Korea, “we’re in for a tough time.”

Mr. Bolton was one of a number of analysts and officials, in and out of the government, who seem alarmed by the havoc they fear Trump and his team may wreak after his inauguration.

A former American ambassador to South Korea and former commander of American forces in the Pacific, Harry Harris, called for “a strong reaffirmation of the Washington Declaration.” That’s the document signed at Washington by President Biden and President Yoon of South Korea in April 2023 calling for a “nuclear consultative group” to talk about defending the South against nuclear attack.

At a Korea Society gathering at Washington, Mr. Harris said North Korea’s leader, Kim Jong-un, “wants to keep his nuclear weapons” and “wants to dominate the Korean peninsula.” He said he is convinced “the time for diplomacy and negotiation with Kim Jong-un is over” and now’s “the time to increase our military readiness.”

On the same panel, the deputy assistant secretary of state for nuclear affairs, Alexandra Bell, said the North Koreans “are pursuing an unprecedented and unchecked nuclear buildup” amid “the growing relationship between Russia and the DPRK,” the Democratic People’s Republic of Korea. The North Koreans, she said, are pumped and primed for their seventh nuclear test — what would be their first since September 2017. “We have a lot of security challenges that require deterrence,” she said.

South Koreans are putting their best face on signs that Trump will want to downplay the Korean-American alliance and possibly cancel joint military exercises, as he did after his first summit with Mr. Kim in Singapore in June 2017, and threaten to reduce the number of American troops in South Korea, already down to 28,500. Almost all are on a few strategic bases, including Camp Humphreys, America’s largest overseas base, 40 miles south of Seoul.

“Burden-sharing, cost-sharing is essential,” a former South Korean defense minister, Lee Jong-sup, said. “With awareness of the DPRK threat, we look forward to further strengthening of the alliance.” A former foreign minister, Yun Byung-se, claimed “very good rapport” with Trump. Now, he said, the aim is to “Make the America Alliance Great Again.”

A now retired former commander of American forces in Korea, General Walter Sharp, took a simplistic view that’s widely seen as impossible: “Either the regime changes or you force the regime to change,” he said. “We need to be able to get information into North Korea,” the world’s tightest dictatorship.

Mr. Bolton saw yet another complication: the tariffs that Trump has said he’ll introduce as soon as he takes office. The targets are said to be China, Canada, and Mexico, for failing to stop illegal immigration and the flow of narcotics into American markets. Trump has also said he wants a 60 percent tariff on Chinese products to redress the enormous Chinese trade surplus.

“Numbers come and go with Trump,” he said when asked about this latest tariff threat. “He’s throwing a lot of dust in the air. He’s throwing out firecrackers.” Yet what if China “during this turmoil decides to cause trouble?” he asked. “It’s potentially a very dangerous time.”