Conscious and planned colonization of the fertile, but underdeveloped region was pioneered in the 1580s and 1590s by the Ruthenian dukes of Volhynia. Of the Poles, only Jan Zamoyski, who penetrated the Bracław area, was economically active by the end of the 16th century. There and in the Kiev area Polish fortunes also began to develop, often through intermarriage with Ruthenian clans. In 1630, the great Ukrainian latifundia were dominated by Ruthenian families, such as the Ostrogski, Zbaraski and Zasławski. At the outset of the great civil war of 1648, the Polish settlers comprised barely 10% of the middle and petty nobility, for example in the well-researched Bracław Voivodeship and Kiev Voivodeship. The early Cossack rebellions were, therefore, instants of social uprising, rather than national anti-Polish movements. As class warfare they were ruthlessly stamped out by the state, which would sometimes take their leaders to Warsaw for execution.