[1][3] The pastoralists arrived in waves and initially founded a number of small kingdoms, exploiting the lack of unity among the already settled farmers.

[1][3] Based on societal and oral traditions, it is generally believed[5] that the region's pastoralists were the ancestors of the later Tutsi ethnic group, while the agriculturalists became the Hutus.

[8] However, Ntare IV's decisions regarding his succession greatly affected the monarchy, as he appointed his sons as the administrators of the newly conquered territories.

[11] The first European explorers to reach Burundi were Richard Francis Burton and John Hanning Speke in 1858, followed by Henry Morton Stanley and David Livingstone in 1871.

[1][6] Mwezi IV initially refused to acknowledge German rule, and demonstratively rejected outside influences, even including cotton clothing.

[13] Besides establishing a military post in the area in 1896, Germany mainly opted to rule indirectly through the mwami, alternating between strengthening and limiting the power of the Burundian monarchy.

[1] Around 1900, an "anti-king" of mysterious origin called Kilima appeared in northern Burundi,[14] using discontent among the local Hutu peasants to challenge the Burundian monarchy and establish his own fiefdom.

However, Burundi suffered under a rinderpest outbreak and sleeping sickness epidemic during this period, causing a substantial loss of population and economic hardship.

[17] From 1914, the Ruzizi River marked the frontline in the area; in September 1915, German forces crossed the border and attacked Luvungi in the neighbouring Belgian Congo.

[19] In May 1916, Belgian troops broke through German defenses in Rwanda as part of the early stages of the Tabora offensive, and then turned south to capture Burundi.

However, their attempt to capture the local German Schutztruppe garrison failed, as Langenn-Steinkeller's force successfully evaded them and retreated from the region.

[17] The Burundian monarchy, at the time led by a regency council due to the minority of mwami Mwambutsa IV, officially surrendered to the Belgians ten days later.

[20] As a result of the atrocities committed in the Congo when it had been directly ruled by Leopold II of Belgium, the population of Burundi was generally fearful of the Belgians; though Force Publique troops did not assuage these fears, as they looted and harassed civilians, the Belgian takeover was mostly orderly and its new regime was initially "no harder (nor any easier)" than that of the Germans.

[21] The war years exerted a heavy toll on Burundi; locals were conscripted as porters and food requisitioned, resulting in many civilian deaths.

[21] In 1922, Burundi was officially assigned to the Belgian colonial empire (together with the neighbouring Kingdom of Rwanda) as part of Ruanda-Urundi, an international mandate by the League of Nations.

The Belgians preserved many of the kingdom's institutions intact,[6][1] but in contrast to the limited overlordship by Germany, they exerted more control, imposing forced labor and more taxes.

[13][3] From this point, the Belgians began to switch their support from the Tutsi minority to the Hutu majority, favoring the latter's takeover of the future state.

[12] The Burundian independence movement was led by Louis Rwagasore, a prince of the Bezi clan[24] and leader of the Union for National Progress (UPRONA).

[28][29] Rwagasore's death derailed his attempts to build national inter-ethnic cohesion and facilitated the growth of Hutu-Tutsi tensions which would dominate the remaining years of the Kingdom of Burundi.

[22] By late 1963, the Burundian government allowed Congolese revolutionary Gaston Soumialot to recruit thousands of fighters along the Burundian-Congolese border.

The following legislative elections resulted in a Hutu majority in the National Assembly, but mwami Mwambutsa IV decided to appoint Léopold Biha,[3] one of his confidants and a Ganwa,[33] as the Prime Minister, hoping to maintain the monarchy's power.

[3] In response, a failed coup d'état was launched against the monarchy by Hutu officers in October 1965; Mwambutsa IV fled the country and refused to return, even as he claimed to still hold supreme power.

[41] Belgian Resident Pierre Ryckmans described the new policy by stating that "the native kings... are the familiar décor that permits us to act behind the scenes without alarming the people".

[42] The Ganwa and Banyamabanga led the native administration which included the local authorities (Batware), delegates (Vyariho), and arbiters (Bashingantahe); these could be Tutsis or Hutus.

[42] The Bashingantahe were important in maintain peace around Burundi; the posts were granted to "outstanding citizens" who guarded traditions, controlled for good behavior, and resolved interpersonal disputes.

[44] The commoners were called Banyagihugu, including all who held no official positions, worked for their basic subsistence, and were required to provide tribute and serve as soldiers in war.

[44] The political and social system of the kingdom was likened to feudalism by researcher Nigel Watt who pointed out that many Hutu peasants, especially in certain provinces, faced substantial repression.

[48] In the 19th century, Islam reached Burundi due to the settlement of Arab and Swahili traders;[49] the Muslims also set up local palm oil production.

In the 1920s, more Protestants like the Seventh-day Adventists, Angelicans, Danish Baptists, Swedish Pentecostals, American Quakers, and Free Methodists came to Burundi.