He was dismissed from the Uganda Army in 1975. He went into exile, first to Nairobi, Kenya, then Dar es Salaam, Tanzania. At some point, he became a member of the militant anti-Amin opposition. Maruru joined the Save Uganda Movement (SUM) which intended to overthrow the Ugandan President through guerrilla attacks, helping to set up the group's command center in Tanzania. When the Uganda–Tanzania War broke out in 1979, Maruru was one of the exiles who organised a pro-Tanzanian militia of Ugandan rebels. As a SUM member, he attended the Moshi Conference which resulted in the merger of several anti-Amin groups into the Uganda National Liberation Front (UNLF) and its armed wing, the Uganda National Liberation Army (UNLA). Maruru was appointed secretary of the UNLA's Military Commission during the conference. At some point during the Uganda–Tanzania War, he became one of the top commanders of the Kikosi Maalum, a contingent of Ugandan rebel troops commanded by Tito Okello and Oyite Ojok.