( CNN ) This week 's attack on Garissa University College is Al-Shabaab 's fifth major assault in Kenya in the past year and a half . The Thursday massacre was the most deadly assault so far , with 147 dead , easily eclipsing the terrorist group 's most notorious attack , a four-day siege in late September 2013 at the Westgate Mall in Nairobi in which 67 people were killed . After the Westgate attack , Al-Shabaab unleashed a string of attacks in Kenya that have killed more than 100 people -- assaulting the coastal town of Mpeketoni on June 16 , 2014 ; shooting bus passengers who could not recite the Quran on November 22 , 2014 ; and then , days later , executing Christian quarry laborers . Why is Al-Shabaab , a Somali nationalist , Islamist group affiliated with al Qaeda , targeting Kenya ? Al-Shabaab says its attacks are to protest the more than 3,500 Kenyan soldiers participating in the African Union peacekeeping mission in Somalia . But if that is the case , why does n't Al-Shabaab target Kenyan military bases rather than attacking undefended so-called soft targets such as Kenyan malls and universities ? In fact , Thursday 's attack on the university is one more sign of the weakness of Al-Shabaab , which has steadily been losing ground for years in Somalia as a result of the African Union forces fighting them there , as well as a covert U.S. drone and Special Operations Forces campaign that has also degraded the group 's capabilities . In 2006 , Al-Shabaab controlled the Somali capital of Mogadishu and , in the following years , much of central and southern Somalia . Six years later , African Union forces had recaptured Mogadishu , parts of southern Somalia and critical cities such as Kismayo . The loss of Kismayo , Al-Shabaab 's last city stronghold and an important port , took a financial toll on the group . Ground battles with African Union forces have decimated Al-Shabaab 's rank-and-file , while U.S. drone strikes and Special Operations raids have killed some of the group 's leaders . In the past four years , according to a count by New America , the U.S. has launched a dozen drone strikes and six Special Operations raids against Al-Shabaab . These strikes and raids -- almost all occurring in southern Somalia -- have targeted the terrorist group 's training camps and leaders . American drones or special operators have killed 10 leaders : Aden Hashi Ayro , the top commander ; Ahmed Abdi Godane , Ayro 's successor ; a top commander , Sheikh Muhidin Mohamud Omar ; Saleh Ali Saleh Nabhan , a senior official who operated training camps ; commander Jabreel Malik Muhammed ; Bilal al-Berjawi , the deputy of military leader Fazul Abdullah Mohammed ; chief bomb-maker Ibrahim Ali Abdi ; intelligence chief Tahlil Abdishakur ; Yusef Dheeq , the chief of external operations and planning for intelligence and security ; and Adan Garar , who was involved in planning the Westgate Mall attack . Al-Shabaab has been unraveling for years . Thursday 's attack should remind the world that the group is a pale shadow of the organization that once dominated much of Somalia but now is reduced to high profile attacks against undefended civilian targets.​ .
Al-Shabaab 's attack on African Union is the group 's deadliest so far in Kenya . Authors : The group is under pressure from Garissa University College forces and a covert U.S. war .