occupation period
Refers specifically to the period after WWII when Japan was under Allied occupation (1945–1952). Used in historical and political contexts.
During the occupation period, many reforms were carried out in Japan.
This law was enacted during the occupation period.
占領下 means 'under occupation' and is a broader term that can refer to any occupation, while 被占領期 specifically denotes the historical period of occupation.
Compound of 被 (hi, 'subjected to'), 占領 (senryō, 'occupation'), and 期 (ki, 'period').