( 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 .
To the latter 's efforts , one must realize that Kenya and Al - Qaeda are at risk . Al Qaeda 's 1998 attack on the U.S. embassy is the single largest , but Al - Shabaab have killed many more Kenyans .