South Korean protesters burn North Korean flags and a photo of a suspect of slashing U.S. Ambassador Mark Lippert, during a rally denouncing the attack and demanding the alliance between South Korea and the United States, in Seoul, South Korea, Thursday, March 5, 2015. AP SEOUL, South Korea (AP) — A knife attack Thursday that injured the U.S. ambassador to South Korea is only 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 was an extreme example, but America infuriates some leftist South Koreans because of its role in Korea's turbulent modern history.

Washington 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, a 55-year-old named Kim Ki-jong 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 right cheekbone and extended diagonally across his cheek toward his jawbone. He received 80 stitches to close that 11-centimeter (4-inch) long, 3-centimeter deep (about an 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.

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 annual U.S.-South Korean military drills that started Monday — exercises that Pyongyang has long maintained are preparations for an invasion. Kim said the drills, which Seoul and Washington say are defensive only, ruined efforts for reconciliation between the two Koreas, officials at Seoul's Jongno police station said in a televised briefing.

Injured U.S. Ambassador to South Korea Mark Lippert, center, gets into a car to leave for a hospital in Seoul, South Korea, Thursday, March 5, 2015. AP Photo/Yonhap, Kim Ju-Sung

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 protest against the U.S. for driving the Korean Peninsula to the brink of war because of the joint military drills.

Police were aware of Kim's history of violent behavior, according to a Seoul police official who didn't want to be named, citing office rules. However, they didn't consider the possibility that Kim, who has ties to the Korean Council for Reconciliation and Cooperation, the group that hosted the breakfast meeting where Lippert was attacked, would show up for the event, said the official.

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 annual U.S.-South Korean military drills, which will run until the end of April.

South Korea's Unification Ministry says that 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.

A suspect, bottom, identified by police as a 55-year-old, surnamed Kim, is overwhelmed by participants at a lecture hall in Seoul, South Korea, Thursday, March 5, 2015. AP Photo/Yonhap, Kim Ju-Sung 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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AP writers Hyung-jin Kim in Seoul Matthew Lee in Washington contributed to this report.

Follow Foster Klug, AP's Seoul bureau chief, at https://twitter.com/APKlug and Kim Tong-hyung athttps://twitter.com/KimTongHyung