In December 2018, Al-Shabab Al-Mujahideen, Al-Qaeda’s branch in Somalia, published an 18-page document calling for battle against members of the Islamic State in Somalia for the crimes it claimed the organization committed in Somalia. The backdrop to the publication was the escalation in relations between the Islamic State and Al-Shabab.
Clashes between the Islamic State in Somalia and Al-Shabab are nothing new and began back in 2015 when some members of Al-Shabab defected from the ranks of the organization and swore allegiance to the Islamic State. The main group of the Islamic State in Somalia operates under the leadership of Abd al-Qadir Mumin, a former Al-Shabab official who appeared in many of the organization's video clips before he defected to join the ranks of the Islamic State in October 2016. However, it appears that while the conflict between Al-Shabab and Mumin’s group focused mainly on assassinations by Al-Shabab of members who defected to the ranks of the Islamic State, Al-Shabab declared an all-out war against the Islamic State in Somalia in a document presented below.
It should be noted that Al-Shabab’s activities against the Islamic State in Somalia in the three years that have passed since then have prevented the latter from spreading, but have not succeeded in completely eradicating the Islamic State’s presence. According to various reports, the Islamic State carried out 96 attacks inside Somalia since April 2016. There is also various evidence that the Islamic State inside Somalia maintains contact with the broad network of the Islamic State. 
It seems that, despite the small number of Mumin’s fighters compared to the number of Al-Shabab fighters, the former succeeded in attracting to its ranks several foreign fighters from Al-Shabab, which led to a battle between a group of foreign fighters belonging to the Islamic State and those remaining loyal to Al-Shabab. For example, violent internal clashes took place within Al-Shabab in November 2018 when the organization's fighters killed one of its foreign fighters, claiming that he had left Al-Shabab and defected to the Islamic State. A similar incident took place in southern Somalia in the beginning of 2017.
 December 16, 2018 https://www.longwarjournal.org/archives/2018/11/islamic-state-warns-shabaab-of-impending-battle-in-somalia.php