This is the 119th most frequent Japanese word.
記念
Commemoration or memory.
Here, '記念' is used in the sense of a memorial or something symbolic to remember an event, in this case, associated with '戦争' (war) to indicate its purpose.
この建物は戦争の記念として保存されています。
This building is preserved as a memorial to the war.
Here, '記念' (kinen) is used in the context of 'anniversary', combined with '日' (day) into '記念日' (kinenbi), meaning 'commemorative day' or 'anniversary'.
結婚記念日に旅行に行きました。
We went on a trip on our wedding anniversary.
In this sentence, '記念' refers to a keepsake or something intended to preserve a memory, combined with '写真' (photo) to form '記念写真' (kinen shashin), meaning 'commemorative photo'.