Final answer:
The Tutsi rebel army formed after 1990 to defend themselves against the genocide orchestrated by Hutu warlords. The Tutsi rebels eventually took control of Rwanda's government, ending the slaughter and causing a mass exodus of Hutus fearing retribution.
Step-by-step explanation:
The formation of the Tutsi rebel army after 1990 and their subsequent actions are rooted in their long-standing ethnic conflict with the Hutu majority in Rwanda. This conflict was exacerbated by historical colonial policies which favored the minority Tutsis during German and later Belgian rule, fueling resentment among the Hutu majority. When Rwanda gained independence in 1962, the power shifted to the Hutus who, after decades of feeling oppressed, started periodic violence against the Tutsis.
In 1994, in response to years of this ethnic tension and violence, a group of Hutu warlords initiated a systematic genocide against the Tutsi population, an atrocity that lasted several months and resulted in the death of as many as one million people. The Tutsis formed a rebel army, mainly composed of Tutsi refugees who had fled previous violence, to defend themselves and to fight against the Hutu-led government responsible for the genocide.
Eventually, the Tutsi rebels gained strength and, with the support of other African nations, managed to seize control of the government by the end of the summer, thereby ending the mass killings. The aftermath of this conflict saw over a million defeated Hutus fleeing Rwanda due to the fear of retribution, leading to a humanitarian crisis in refugee camps across East Africa.