The overnight attack on a hotel complex in Nairobi that left 14 dead was quickly claimed by the Islamic extremist group al-Shabaab, which has targeted Kenya with several devastating assaults in recent years, leaving hundreds dead. Here's a look at the group which has been called the deadliest in Africa:WHAT IS AL-SHABAAB?Al-Shabaab, meaning "the youth" in Arabic, emerged in neighboring Somalia more than a decade ago as the chaotic Horn of Africa country was deep in warlord-led fighting. The extremist group, linked to al-Qaeda, has been fighting to establish an Islamic state in Somalia based on Shariah law. Its members are mostly Somalis but include many foreign fighters. Recently al-Shabaab has fought a splinter group of fighters who have claimed allegiance to the Islamic State organization.Al-Shabaab is fighting Somalia's fragile central government, which is supported by a multi-national African Union force that expects to pull out in the next few years and leave security to Somalia's military. The extremists once controlled large parts of Somalia, including most of Mogadishu, the capital, but a concerted effort by the AU and Somali forces in 2015 pushed al-Shabaab out of most urban centers. The group still operates in large swathes of rural Somalia and mounts violent suicide attacks on high-profile targets such as hotels and checkpoints in Mogadishu.Al-Shabaab is blamed for Somalia's deadliest attack, a massive truck bomb in Mogadishu in 2017 that killed well over 500 people.WHY DOES AL-SHABAAB ATTACK KENYA?Al-Shabaab has vowed retribution after neighboring Kenya sent troops into Somalia in 2011 to battle the extremists. The Kenyans' deployment came after al-Shabab kidnapped tourists in Kenya's popular Lamu resort area on the Indian Ocean near the Somali border, a blow to the lucrative tourist industry.Tuesday's violence came three years to the day after al-Shabab attacked a Kenyan military base in Somalia, killing scores of people.Al-Shabab has staged several attacks inside Kenya, including the 2013 attack on Nairobi's Westgate Mall in which 67 people were killed and the 2015 attack on Garissa University in which 147 people, mostly students, were killed. The extremists have also targeted schools and bus transport near the porous border with Somalia, at times singling out Christians.This latest attack occurred a short distance from Westgate Mall and again appeared to target wealthy Kenyans and expatriates.ARE US AIRSTRIKES WEAKENING AL-SHABAAB?Shortly after US President Donald Trump took office in early 2017, he vowed to step up military action against al-Shabaab. Last year the US carried out nearly 50 airstrikes, some targeting top extremist leaders. The airstrikes may pressure al-Shabaab to stay on the move, but the extremists are still able to renew their ranks.Al-Shabab continues to operate in rural areas across Somalia, enriching itself with a widespread system of "taxation" on travelers and cargo. It now competes with the new IS-linked fighters in extorting Somali businesses.The latest Nairobi attack used "almost identical tactics" to al-Shabaab's frequent attacks on hotels in Mogadishu, said Matt Bryden of Sahan Research, an expert on the extremists. The airstrikes hamper the group but have not "seriously degraded al-Shabaab's capability to mount strikes either inside or outside Somalia," Bryden said. Airstrikes alone cannot defeat the extremists, he said, and must be combined with more ground-based attacks as well as a non-military campaign to win over residents of extremist-held areas.