Complex sentence: The drastic demographic reduction of the Charrúa did not occur until the first president of Uruguay Fructuoso Rivera, organized a genocide campaign known as "La Campaña de Salsipuedes" in 1831.
Split into simpler sentences: The drastic demographic reduction of the Charrúa did not occur until the first president of Uruguay Fructuoso Rivera. He therefore organized a genocide campaign known as "La Campaña de Salsipuedes" in 1831.