Complex sentence: Muwanga had briefly been the "de facto" President of Uganda from May 12-20,1980, as one the third of three presidents who served for short periods of time between Amin's ouster and the setting up of the Presidential Commission.
Split into simpler sentences: Muwanga had briefly been the "de facto" President of Uganda from 12 May to 20 May in 1980. Muwanga was the third of three Presidents who served for short periods of time between Amin's ouster and the setting up of the Presidential Commission.