That same year, Rwagasore established a nationalist political movement, UPONRA (Union for National Progress). Believing that the role of the royal family should transcend partisan politics, his father promoted him to Chief of Butanyerera, but Rwagasore turned down the appointment so that he could devote himself fully to the nationalist cause. At the first UPRONA Congress (March 1960), he demanded complete independence for Burundi and called on the local population to boycott Belgian stores and refuse to pay taxes. Because of his calls for civil disobedience, he was placed under house arrest.
Despite the setbacks, Rwagasore and UPRONA won a clear victory in elections for the colony's Legislative Assembly in 1961, winning 80 percent of the vote. The next day, he was declared prime minister, with a mandate to prepare the country for independence. Just two weeks later, he was assassinated by his political rivals, the pro-Belgian PDC. He died eight months before seeing Burundi gain its independence. Inter-ethnic rivalries between the Hutu and Tutsi factions of UPRONA flared shortly after. With Rwangasore's death came the end of his dream of a united, democratic Burundi.