text: (CNN)When Al-Shabaab gunmen slaughtered 147 people at a college campus in Garissa, Kenya, on Thursday, it was the bloodiest blow Islamist extremists have dealt to the country since the 1998 bombing of the U.S. Embassy in Nairobi. But it was also yet another pave stone in the long path of violence between Kenya and the Somali militants. To understand Thursday's attack, one must understand this: Kenya and Al-Shabaab are at war. Although al Qaeda was behind the 1998 attack that killed more than 200 people, Al-Shabaab has killed many more Kenyans and is by far the country's most persistent tormenter, according to a University of Maryland study. Their attacks have notched up since 2008. And in October 2011, Kenya Defense Forces invaded Somalia, where Al-Shabaab is based, with the crushing Operation Linda Nchi, Swahili for "Protect the Country." Thus began a bloody vicious cycle, with Al-Shabaab retaliating on Kenyan soil with ever more spectacular mass killings. Here are some of the most heinous attacks on Kenya by the Somali terrorist group. Early Thursday, while many students still slept, at least four gunman burst into a Christian prayer service at Garissa University College, leading into Easter Weekend. They shot students and took hostages, whom they herded across campus. As they encountered students, they separated them into Muslims and Christians. They spared the former, eyewitness Joel Ayora said. They killed the latter. Kenyan special forces moved in at the campus located about 90 miles from Somalia's southern border. They killed four gunman, but not before 147 victims were shot dead. In December, Al-Shabaab militants carried out the same macabre division of captives by religion at a rock quarry not far from the Somali border. They shot Christian workers dead, and spared their Muslim colleagues. Red Cross Workers counted at least 36 bodies dumped into the quarry. It was eye-for-an-eye retaliation, the militants said, for raids Kenyan security forces carried out on mosques intended to weed out extremists. Parallel to Operation Linda Nchi, Kenya has gone after ethnic Somalis within their own borders triggering protest by Human Rights Watch, which has accused the government of ethnic profiling and mistreatment. In a 2011 case, Kenyan authorities reportedly rounded up ethnic Somalis in Garissa and let them sit in the mud, while beating some of them, HRW alleged. Passengers on a bus in Kenya's north were told to recite from the Quran or die in November 2014. Shortly after the bus carrying some 60 people departed Mandera, near Somalia's border, Al-Shabaab gunmen stopped it in a hilly area and barged in. Those who failed to recite verses were gunned down, leaving 28 dead. In its claim of responsibility, Al-Shabaab said the dead were Christians, and their killing was retaliation mosque raids by Kenyans. A day after the attack, Kenyan security forces retaliated with an offensive across the border into Somalia. They claimed to kill 100 Al-Shabaab militants. In July, Al-Shabaab gunmen opened fire on the Lamu coast, a popular tourist region, and at a police station in a neighboring county.  They killed 22 people. Last May, tour companies evacuated vacationers from Kenya after the UK and United States issued alerts that there was a high threat of attacks. Before the Garissa massacre, the Westgate Mall attack was the deepest wound Al-Shabaab had inflicted on Kenya. For four days, four attackers turned the luxury shopping mall in the capital Nairobi into a cauldron of blood, smoke and rubble. Part of the mall collapsed as Kenyan security forces fought for control. At least 61 civilians were killed, as well as a handful of security officers. Al-Shabaab said the attack was retaliation for Kenya participation in Africa Union military operations in Somalia. Even with Al-Shabaab's shooting in Garissa, al Qaeda's bombing of the U.S. Embassy in 1998 remains the single bloodiest Islamist terror attack on Kenyan soil. In short succession, bombs detonated at U.S. embassies in Kenya and Tanzania back then. The Nairobi bombing was much deadlier that the explosion at Dar es Salaam. Al Qaeda founding member Mamdouh Mahmud Salim was arrested and accused in the bombing and is being held in a New York jail, where he still awaits trial. He is serving a 32 year sentence after being convicted of stabbing a prison guard there in the eye.
target: To understand Thursday's attack, one must understand that Kenya and Al-Shabaab are at war .
  Al Qaeda's 1998 attack on the U.S. Embassy is the single largest, but Al-Shabaab have killed many more Kenyans .