Before contact with Europeans, Rwanda and Burundi were kingdoms (primarily inhabited by Hutu, Tutsi and Twa) competing to gain control over nearby territory.

[6][7][4] Around the early-to-mid 19th century, Rwanda and Burundi engaged in the conquering and annexing of smaller surrounding kingdoms, reaching their greatest territorial extent.

[10][11] Although Belgium initially continued the German method of government through the monarchy, in 1926, it began a policy of direct colonial rule in line with norms in the Belgian Congo.

Belgian reforms also extended uburetwa (forced labour by Hutu for Tutsi chiefs) to individuals, not just communities, and to regions not previously covered by the system.

[15] Ruanda-Urundi was devastated by a famine during World War II; a combination of drought and the trade policies of colonial authorities caused between 36,000 and 50,000 people to die in 1943–1944,[16][17] with hundreds of thousands more fleeing to neighbouring areas.

Upon learning of this, United Nations Commission on Human Rights (UNCHR) Representative in Bujumbura Jacques Cuenod, along with a group of Protestant missionaries, alerted the Burundian government and tried to persuade them to stop the attack.

One refugee later told UNCHR official Francois Preziosi that François Rukeba, the Rwandan government-in-exile's Minister of Defence,[27] had ordered the attack after a meeting in Bujumbura, during which Tutsi rebel leaders from other countries expressed their opposition.

Burundian Vice Prime Minister Pié Masumbuko told a Rwandan official, "Recently we have arrested people who were about to attack you and now you decide to sever economic relations with us.

Burundian Prime Minister Pierre Ngendandumwe denied his government's involvement in the invasion,[40][a] and attempted to convince the Organisation of African Unity (OAU) to convene a meeting on the apparent border incident.

Despite these arrests and subsequent investigations,[46] the Supreme Court of Burundi dismissed all charges against those accused in the assassination in December 1967, citing a lack of evidence.

[56] These killings (called the Ikiza)[57] prompted a large, mostly-Hutu exodus from Burundi to neighboring countries, including around 6,000 to Rwanda (although roughly half would later leave for Tanzania).

[65] Rwanda condemned the 1993 coup attempt in Burundi,[66] which killed the Hutu president Melchior Ndadaye[67] and became a catalyst for widespread ethnic violence against Burundian Tutsi.

[74][75][b] Radio Télévision Libre des Mille Collines (RTLM), a Rwandan Hutu extremist propaganda station, deliberately misreported the details of Ndadaye's death – saying he had been tortured and castrated – to inflame anti-Tutsi sentiment.

The Burundian government declared that the plane crash was caused by an accident,[82] and President of the National Assembly Sylvestre Ntibantunganya made a broadcast on television appealing for calm.

[95] The military and paramilitary forces of the old Rwandan Hutu regime (Ex-FAR/ALiR and Interahamwe) subsequently fled into Zaire, then rebuilt their strength and launched an insurgency against the RPF.

The RPF feared that the collapse of the Burundian government would lead not only to the influx of possibly 500,000 Tutsi refugees into Rwanda, but also provide a new haven to the Rwandan Hutu insurgents.

Rwandan troops would repeatedly cross the border and attack Hutu refugee camps which harbored rebel forces, in coordination with the Burundian military and local Tutsi militias.

By overthrowing Zairian President Mobutu Sese Seko, Rwanda hoped to eliminate Zaire as a haven for various Hutu rebel groups.

[107][108] Beginning in April 2015,[109] and especially after a failed coup attempt the following month,[110][111] Burundi (now led by the CNDD–FDD) launched major crackdowns on protesters, prompting more than 390,000 Burundians to flee to neighbouring countries by May 2018, including to Rwanda.

[112] The unrest caused even more strain on relations between Rwanda and Burundi; Kagame was among those critical of Nkurunziza for seeking a third term, and both countries accused each other of harbouring hostile rebel groups.