[2] His brother Wilson "Willy" Kimumwe also joined the military, eventually becoming a pilot in the Uganda Army Air Force[3][4] and training to fly MiG-21 and MiG-17 jets in the Soviet Union and Iraq.

In the next months, their conspiracy grew to include 500 Uganda Army soldiers, mostly Christian Baganda and Basoga serving in the air force and Malire Battalion.

[5] The ULM launched their coup attempt, code-named "Operation Mafuta Mingi" on 18 June 1977, but the plan was leaked to the State Research Bureau, Uganda's intelligence agency.

[5] Even though some conspirators, including Wilson Kimumwe, were subsequently able to escape into exile,[4] many ULM members were captured by the security forces.

[11] The group began to work on one ventilator, using the scrap in their cell to create tools with which they could silently cut the wire gauze and remove the glass, succeeding in this by 11 September.

[12] One day, Kimumwe, Mutumba, Kassujja, Ssekalo were temporarily removed from their cell and brought to Major Faruk Minawa, the SRB's operations officer.

On 20 September, the four were again removed from their cell and transported to one of President Amin's residences, where Faruk informed them that their behavior might decide whether they would be spared or executed.

[16] Amin threatened that they would face a military tribunal soon, and if found guilty, blown up by the same bazookas which the dissidents had imported for the coup attempt.

[17] Even though the President claimed that they would be spared if they pleaded for mercy, the four knew that almost all military tribunals chaired by Juma Butabika ended in death sentences.

They made their first attempt to get past the metal bars in the night of 20 September,[19] but the opening was still too small for the two largest inmates, Ssekalo and Okech.

[2] Kimumwe and the other six prisoners managed to get into the open, and began to carefully crawl around, hoping to find some way to leave the SRB compound.

[2][24][c] Following their escape, the SRB launched new purges and ordered the arrest as well as murder of several people who were suspected of connections to Operation Mafuta Mingi.

[2] Kimumwe told the Drum magazine that the two hoped to inform people about the true nature of Amin's regime, arguing that "his clowning conceals a ruthless extinction of human rights" in Uganda.

[9] Kimumwe became part of the militant anti-Amin opposition in Kenya,[2] and eventually met Yoweri Museveni who invited him to join the Front for National Salvation.