Complex sentence: Before it became the government in 1991, the EPRDF was a rebel group battling the Derg, a military regime led by Mengistu Haile Mariam that was effectively in power from 1974 until it was ousted by the EPRDF in 1991.
Split into simpler sentences: Before it became the government in 1991, the EPRDF was a rebel group battling the military regime known as the Derg. The Derg, led by Mengistu Haile Mariam, was effectively in power from 1974 until it was ousted by the EPRDF in 1991.