Vladimir had been prince of Novgorod when his father Sviatoslav I died in 972. He was forced to flee to Scandinavia in 976 after his half-brother Yaropolk had murdered his other brother Oleg and taken control of Rus. In Scandinavia, with the help of his relative Earl Håkon Sigurdsson, ruler of Norway, Vladimir assembled a Viking army and reconquered Novgorod and Kyiv from Yaropolk. As Prince of Kyiv, Vladimir's most notable achievement was the Christianization of Kievan Rus', a process that began in 988.