noun
occupied country
A country under military occupation by a foreign power.
戦後、日本は被占領国となった。
After the war, Japan became an occupied country.
占領国 is the occupying power, while 被占領国 is the country being occupied.
Compound of 被 (hi, 'receiving' or 'passive marker'), 占領 (senryō, 'occupation'), and 国 (koku, 'country').