Although missionaries introduced some of their customs to Buganda and some were adopted, the custom of the firstborn son of the Kabaka succeeding his father never caught on. The Bazungu tried it, and it backfired.
Kabaka Mwanga II’s brother, Kiweewa, proclaimed himself Kabaka during the time Mwanga had been exiled in the latter years of the 19th century. The Bazungu were behind this because Mwanga was not cooperative with them.
The religious battles of 1884-1897 led to armed conflicts that saw Mwanga’s brothers Kiweewa and Kalema each trying to dethrone Mwanga. They allied themselves with whichever religion they thought was strong.
Mwanga was overthrown and exiled with Kabalega in the Seychelles 🇸🇨. That led to Kiweewa and Kalema becoming Kabakas for short periods each until 1897, when the one-year-old Daudi Chwa, Mwanga’s son, was proclaimed Kabaka. The Baganda opposed Kiweewa and Kalema’s reigns.
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So why doesn’t Kiweewa, the firstborn son of the Kabaka, become Kabaka? There is no crown prince system in Buganda like with the British or Saudis. This is because the Baganda thought that automatic succession could destabilize the kingdom.
If a Kabaka lived a long life, the Baganda thought, the crown prince could kill him so that he could take over. So they opened up succession to any other son apart from the Kiweewa.
However, the reigning Kabaka can make changes to any norms as he may wish depending on prevailing circumstances. So it is possible that a Kiweewa can become Kabaka in the future.
In fact, some people had expected Prince George Mawanda, a Kiweewa, to become Kabaka instead of Muteesa II because their father Daudi Chwa II had accorded him special treatment. However, Chwa II told all his children that he preferred Muteesa to inherit him.
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The reasons Chwa II gave were based on religion, arguing that since Mawanda’s mother was not the church-wedded one, the Bazungu would fight him and further create more conflicts in the kingdom. Mawanda ended up as his brother’s ADC since he was a lieutenant in the Uganda army.
When Muteesa II was exiled in Britain in 1953-1955, the British tried to install Mawanda as the Kabaka, and even Obote toyed with the same idea after the 1966 crisis. But the Baganda were not open to that idea, after all, Muteesa II was still alive.