SEOUL >> 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. Lippert, 42, also had surgery on his arm to repair damage to tendons and nerves and was in stable condition at the hospital.

About nine hours after the attack, Lippert posted on his Twitter account that he was “doing well and in great spirits” and would be back “ASAP” to advance the U.S.-South Korean alliance.

U.S. Secretary of State John Kerry, in Saudi Arabia for meetings with regional leaders, said the attack would not reduce America’s resolve in pursuing its interests.

“The United States of America will never be intimidated or deterred by threats or by anybody who harms any American diplomats,” he said.

The State Department said it could not speculate on a motive at this time, and that South Korean President Park Geun-hye had called Lippert in the hospital to express her condolences.

Kim is well-known among police and activists as one of a hard-core group of protesters willing to use violence to highlight their causes. Such protesters often speak of their actions in terms of a war, of a struggle to the death.

Kim told police that he attacked Lippert to protest U.S.-South Korean military drills that started Monday — exercises that the North has long maintained are preparations for an invasion. Kim said the drills, which Seoul and Washington say are purely defensive, ruined efforts for reconciliation between the two Koreas, officials at Seoul’s Jongno police station said in a televised briefing.

North Korea’s state-controlled media later crowed that Kim’s “knife slashes of justice” were “a deserved punishment on war maniac U.S.” and reflected the South Korean people’s protests against the U.S. for driving the Korean Peninsula to the brink of war because of the joint military drills.

Police didn’t consider the possibility that Kim, who has ties to the Korean Council for Reconciliation and Cooperation, which hosted the breakfast meeting where Lippert was attacked, would show up for the event, according to a Seoul police official who didn’t want to be named, citing office rules.

U.S. ambassadors have security details, but their size largely depends on the threat level of the post. Seoul is not considered to be a particularly high threat post despite its proximity to the North Korean border. It’s not clear how many guards Lippert had, but they would have been fewer than the ambassadors in most of the Mideast.

Seoul’s Foreign Ministry said it was the first time a foreign ambassador stationed in modern South Korea had been injured in a violent attack.

However, the Japanese ambassador narrowly escaped injury in 2010 when Kim threw a piece of concrete at him, according to police. Kim, who was protesting Japan’s claim to small disputed islands that are occupied by South Korea, hit the ambassador’s secretary instead, media reports said, and was sentenced to a three-year suspended prison term over the attack.

The website of the Woorimadang activist group that Kim heads describes the group’s long history of anti-U.S. protests. Photos show him and other activists rallying last week in front of the U.S. Embassy in Seoul to protest the U.S.-South Korean military drills, which are to run until the end of April.

South Korea’s Unification Ministry says Kim visited North Korea with a civic group eight times between 2006 and 2007, during a period of inter-Korean cooperation under a liberal government in Seoul.

In a Facebook posting, Yoon Meehyang, who heads an activist group representing South Korean women forced to serve as wartime sex slaves for Japanese troops, said Kim was a consistent trouble-maker at the group’s rallies several years ago. Yoon and other activists expressed worries that the attack on Lippert would harm the public image of peaceful leftist protesters, or prompt the conservative government to suppress their activities.

Small to medium-sized demonstrations regularly occur across Seoul, and most are peaceful.

But scuffles with police do break out occasionally, and the burning of effigies of North Korean and Japanese leaders is also common. Some demonstrators have also previously severed their own fingers, thrown bodily fluids at embassies and tried to self-immolate.

Lippert became ambassador last October and has been a regular presence on social media and in speeches and presentations during his time in Seoul. His wife gave birth here and the couple gave their son a Korean middle name.

He will need treatment at the hospital for the next three or four days and may experience sensory problems in his left hand for several months, said Chung, the hospital official.

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Associated Press writers Hyung-jin Kim in Seoul and Matthew Lee in Riyadh, Saudi Arabia, contributed to this report.