The resulting nationwide campaign of Hutu violence against Tutsis becomes known as 'the wind of destruction'.
Over the coming months many Tutsis flee from Ruanda, including the 25-year-old hereditary ruler, the Mwami.
In Urundi the Tutsi monarchy proves at first more resilient, both in holding on to the reins of power and in attempting a resolution of the Tutsi-Hutu conflict.
When elections are held in 1961, they bring a landslide victory for a joint Hutu and Tutsi party.
The Hutu are subject to the forced labour which disfigures many European colonies in Africa, but here it is the Tutsi who supervise them at their tasks.
From 1933 everyone in Ruanda-Urundi is issued with a racial identity card, defining them as Hutu (85%) or Tutsi (14%).
After the war the League of Nations confirms the existing state of affairs, granting Belgium in 1924 a mandate to administer the colony.
His realm is organized on a feudal basis, with the Tutsi as the aristocracy and the Hutu as their vassals.
The highlands of Rwanda and Burundi, east of Lake Kivu, are the last part of Africa to be reached by Europeans in the colonial expansion of the late 19th century.
Before that time local tradition tells of many centuries during which the Tutsi, a tall cattle-rearing people probably from the upper reaches of the Nile, infiltrate the area and win dominance over the Hutu, already in residence and living by agriculture.
The remaining 1% are the Twa, the remnants of the original Pygmies indigenous in this area.
This Belgian attitude, setting in stone the distinction between the two groups and favouring one of them, prepares the ground for future violence (in earlier times racially based massacres have never occurred between Hutu and Tutsi).