(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 doesn'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 Garissa University College is the group's deadliest so far in Kenya .
Authors: The group is under pressure from African Union forces and a covert U.S. war .