Complex sentence: During his absence, Rwigyema had joined the rebel army of Yoweri Museveni, which was based in Tanzania and aimed to overthrow the Ugandan government of Idi Amin.
Split into simpler sentences: During his absence, Rwigyema had joined the rebel army of Yoweri Museveni. Based in Tanzania, it aimed to overthrow the Ugandan government of Idi Amin.