SEOUL, SOUTH KOREA - Former U.S. Defense Secretary Jim Mattis on Friday lobbed insults at Russian President Vladimir Putin and slammed his invasion of Ukraine as "incompetent" and "foolish."

At a speech in Seoul, Mattis compared Putin to the kind of paranoid characters created by Russian novelist Fyodor Dostoevsky.

"Putin is a creature straight out of Dostoevsky. He goes to bed at night angry, he goes to bed at night fearful, he goes to bed at night thinking Russia is surrounded by nightmares," Mattis said.

Mattis was speaking at a forum organized by the Seoul Forum for International Affairs, the Korea Society, and the Center for Strategic and International Studies. He has made relatively few public comments since resigning as Pentagon chief in 2018 over a foreign policy disagreement with former U.S. President Donald Trump.

In his speech, Mattis did not address those disagreements in a direct way, saying only Trump had overseen a nontraditional foreign policy that had challenged U.S. relations with its allies.

Mattis' most pointed comments focused on Putin, whom he portrayed as unhinged and unable to make smart decisions due to the lack of people giving him sound advice.

Asked about the biggest lesson that could be drawn from Russia's war in Ukraine, Mattis replied, "Don't have incompetent generals in charge of your operations."

He also said the Russian invasion was "tactically incompetent' and "strategically foolish."

Russian President Vladimir Putin attends a meeting on the sidelines of the Caspian Sea littoral states summit in Ashgabat, Turkmenistan, June 29, 2022.

"War is enough of a tragedy without adding stupidity on top," he said.

Mattis also criticized China's growing relations with Russia and its unwillingness to oppose the war in Ukraine.

A country "cannot be great if they support Russia's criminal invasion of Ukraine," he said.

Addressing his tenure under Trump, Mattis spoke of "raucous times" and called Trump an "unusual leader" but did not directly criticize the former president.

"Democracies will at times go popularist and will at times break with tradition," he said. "It's the nature of democracies at times to be testing ideas and all."

Americans, Mattis said, should respond by "keep[ing] faith in the institutions" and "in those that disagree with you."

Mattis' speech was in South Korea, a U.S. ally that dramatically felt the effects of Trump's nontraditional foreign policy.

Asked how he felt about Trump's summits with North Korean leader Kim Jong Un, Mattis said he was never optimistic about the talks, but that the diplomatic effort was the "right thing to do."

"As far as what came out of it, nothing. I saw nothing that came out of it," he said.

Mattis also praised South Korea's new president, former chief prosecutor Yoon Suk Yeol, for wanting South Korea to play a bigger role in the world.

Yoon, a conservative who has explicitly embraced the United States, has said he wants South Korea to become a "global pivotal state." This week, Yoon attended the NATO summit in Madrid, the first time a South Korean leader had attended such a meeting.

Mattis praised Yoon's presence at the NATO summit, saying "a globally pivotal state in South Korea is in all our best interests."

He warned, however, against voices in Seoul who have recently called for South Korea to acquire its own nuclear weapons.

"You don't need nuclear weapons on the peninsula to ensure an extended deterrence so long as there is trust between the ROK and the United States," he said, referring to an abbreviation of South Korea's official name, the Republic of Korea.

Opinion polls consistently show that most South Koreans support their country acquiring their own nuclear weapons, especially as North Korea continues developing its own arsenal.

As a candidate, Yoon said he would ask the United States to agree to a nuclear weapons sharing arrangement, or to redeploy tactical nuclear weapons that Washington withdrew from South Korea in the early 1990s -- notions quickly rejected by the U.S. State Department.

To avoid such an outcome, the United States and South Korea should continue to build trust, including by demonstrating "extended deterrence" against North Korea's nuclear weapons, Mattis said.

"I think anything you can do to avoid having these weapons yourselves, you should do. They are horrible weapons," he said.