과거사가 · 과거사를
past history (especially painful or unresolved past events)
Example
두 나라는 과거사 문제로 오랫동안 갈등을 겪어 왔어요.
The two countries have long been in conflict over issues of their past history.
Word parts
과 pass+거 go+사 history
Literally 'history of what has passed'.
Good to know
In politics it usually means unresolved historical wrongs, as in 과거사 청산 or 과거사 반성.