The Guarani War  of 1756, also called the War of the Seven Reductions, took place between the Guaraní tribes of seven Jesuit Reductions and joint Spanish-Portuguese forces. It was a result of the 1750 Treaty of Madrid, which set a line of demarcation between Spanish and Portuguese colonial territory in South America. The boundary drawn up between the two nations was the Uruguay River, with Portugal possessing the land east of the river. The seven Jesuit missions east of the Uruguay River, known as the Misiones Orientales, were to be dismantled and relocated on the Spanish western side of the river. The seven missions were called San Miguel, Santos Angeles, San Lorenzo Martir, San Nicolas, San Juan Bautista, San Luis Gonzaga, and San Francisco de Borja. These missions were some of the most populous in South America with 26,362 inhabitants, according to a Jesuit census, and many more in the surrounding areas. In 1754 the Jesuits surrendered control of the missions, but the Guarani led by Sepé Tiaraju, refused to comply with the order to relocate. Efforts by the Spanish army in 1754 to forcefully remove the Guarani from the missions failed. On February 10, 1756, a combined force of 3,000 Spanish and Portuguese soldiers fought the Guaraní at the battle of Caiboaté. It resulted in the death of 1,511 Guarani, while the Europeans suffered only 4 deaths. In the aftermath of the battle, the joint Spanish-Portuguese army occupied the seven missions. Eventually Spain and Portugal annulled the 1750 treaty in the Treaty of El Pardo , with Spain regaining control over the seven missions and its surrounding territory.. Who fought the Guarani tribes of seven Jesuit Reductions in the War of the Seven Reductions?