【淪陷】in most case will be translated as: Falls to the enemy’s hand.
If you want to use one word to represent it, I suggest you to use the following word: ” occupied” . Many historian use this word to represent the period of time when Hong Kong is under the hand of Japan.
eg. Hong Kong was occupied by Japan during the world war II.
Throughout the Japanese occupation, Hong Kong was ruled as a detained terrain and was subjected tomartial law.
Be occupied by the enemy; fall into enemy hands∶領土或國土被敵人占領或陷落在敵人手里,常指被敵占領一段長的時間
淪陷 fall to enemy occupation