In a series of demonstrations at the headquarters of Indonesia's armed forces, troops reportedly drank snake blood, walked on fire, smashed bricks with their heads, and rolled their bodies in broken glass.
Mr Mattis - whose own nickname is "Mad Dog" - seemed impressed by the show, which also saw police dogs leaping from helicopters to attack several people during a simulated terrorist crackdown.
The United States delegation looked on as the men then killed live snakes, including cobras, and served their blood to each other in a sign of brotherhood. "The way they were whipping them around - a snake gets exhausted very quickly", he told reporters as he flew to Vietnam.
The U.S. Defense chief looked on as camouflaged men wearing war paint took part in elaborate and at-times unsafe feats of strength and skill.
"The unity of the United Nations on this issue shows that this is a broadly-based assessment of (North Korea)", Mattis said, adding the North is a country the global community doesn't want to possess a nuclear capability.
"You could imagine how much training went into each individual there, that they were able to do that", he said.
His next stop is Vietnam, where authorities will have trouble following this act, writes Adam K. Raymond. Suharto was later deposed in 1998.
Cooperation between the two countries all but collapsed in the 1990s amid evidence of widespread human rights abuses, forcing the U.S.to curb the relationship.
Secretary of Defense James Mattis, shown here speaking in Washington on Friday, will be in Hawaii to meet with the South Korean defense minister. "(It) makes it easier for both of us on the travel side", Mattis said.
Mattis arrived Sunday in Indonesia on his first overseas trip since debuting a strategy to refocus defense efforts on big-power militaries, calling on the military to keep pace with resurgent Russian Federation and China by building relationships in key regions like Southeast Asia. Mattis repeated the name on Tuesday alongside Ryacudu, a signal the USA intends to stand by Indonesian efforts to repel Chinese excursions into its waters.
Mattis, who also met with Indonesian President Joko Widodo on Wednesday, will visit Vietnam next to hold similar talks.
Vietnam has one of the region's fastest-growing economies, he said.