Complex sentence: Ali Mahdi Muhammad (born 1938) was president of Somalia from January (when Mohammed Farah Aidid removed the then president Mohamed Siad Barre) to November 1991, but was not able in that time to exert control over the country.
Split into simpler sentences: Ali Mahdi Muhammad (born 1938) was president of Somalia from January 1991 to November 1991. However, Muhammad was not able in that time to exert control over the country.