Rubuta ci gaba don wannan sakin layi - Lokacin da sarki Monivong ya mutu a ranar 23 ga Afrilu 1941 Gwamna Janar na Indochina na Faransa, Jean Decoux, ya zaɓi Sihanouk ya gaje shi. Kwamitin Sarauta na Kambodiya ya sanya Sihanouk a matsayin sarki a rana mai zuwa, kuma an yi bikin nadin sarautarsa a ranar 3 ga Mayu, 1941. A lokacin da Japan ta mamaye Cambodia
, ya sadaukar da mafi yawan lokacinsa ga wasanni, yin fim, da kuma balaguron balaguro zuwa ƙauye. A watan Maris na 1945 sojojin Japan, waɗanda suka mamaye Kambodiya tun watan Agusta 1941, sun rushe gwamnatin mulkin mallaka ta Faransa. A karkashin matsin lamba daga Jafananci, Sihanouk ya ayyana 'yancin kai na Cambodia kuma ya dauki matsayin firaminista yayin da yake aiki a matsayin sarki a lokaci guda.