Sihanouk became king during French colonial rule in 1941 upon the death of his maternal grandfather, King Monivong. After the Japanese occupation of Cambodia during World War II, he secured Cambodian independence from France in 1953. He abdicated in 1955 and was succeeded by his father, Suramarit, so as to directly participate in politics. Sihanouk's political organization Sangkum won the general elections that year and he became prime minister of Cambodia. He governed it under one-party rule, suppressed political dissent, and declared himself Head of State in 1960.