SEOUL, South Korea — A knife attack Thursday that injured the U.S. ambassador to South Korea is the latest act of political violence in a deeply divided country where some protesters portray their causes as matters of life and death.

The slashing of Ambassador Mark Lippert’s face and arm, which left deep gashes and damaged tendons and nerves, was an extreme example, but America infuriates some leftist South Koreans because of its role in Korea’s turbulent modern history.

Washington, which backed the South during the 1950-53 Korean War against the communist North, still stations nearly 30,000 troops here and holds annual military drills with Seoul. That’s something anti-U.S. activists view as a major obstacle to their goal of an eventual reunification of the rival Koreas.

Purported U.S. interference in Korean affairs appeared to be the main grievance of the man police named as the assailant, Kim Ki-jong, 55, who has a long history of anti-U.S. protests.

“South and North Korea should be reunified,” Kim shouted as he slashed Lippert with a 25-centimeter (10-inch) knife, police and witnesses said.

The attack left a gash on Lippert’s face that started under his cheekbone and extended diagonally across his cheek toward his jawbone. He received 80 stiches to close the 11-centimeter (4-inch) wound, Chung Nam-sik of Severance Hospital told reporters.