開戦（かいせん、kaisen）
- outbreak of war

The Japanese word 開戦かいせん, which is written with the kanji 開かい (meaning "to open" or "to start") and 戦せん (meaning "war" or "battle"), means "the start of war," "the commencement of hostilities," or "the outbreak of war." It signifies the formal beginning of armed conflict between nations or groups. It's a key term used in historical contexts, international relations, and news reporting. It implies a deliberate and official act initiating a period of warfare.

⚫︎ 日本は真珠湾で開戦した。 Translation: Japan initiated the war at Pearl Harbor. ⚫︎ 開戦の日は、歴史の転換点だった。 Translation: The day the war began was a turning point in history. ⚫︎ 両国は開戦の危機に瀕している。 Translation: The two countries are on the brink of war.

This word is typically used in formal contexts, such as news reports, historical discussions, academic papers, and official government communications. It describes the precise moment when a state of war officially begins. It's not usually used in everyday casual conversation. You would use 開戦かいせん to describe the act of initiating a war, not the war itself. For that, you would use words such as 戦争せんそう (war).

The word is written as 開戦かいせん using the kanji 開かい and 戦せん. The reading is always "かいせん" in hiragana. There are no variations in the okurigana, as there are no hiragana characters after the kanji characters.

⚫︎ 戦争の開始せんそうのかいし (the start of war) ⚫︎ 交戦こうせん (outbreak of hostilities, engagement) ⚫︎ 戦闘開始せんとうかいし (start of combat)

⚫︎ 停戦ていせん (ceasefire, armistice) ⚫︎ 終戦しゅうせん (end of war) ⚫︎ 和平わへい (peace)

その国の政府は、突然の開戦に衝撃を受けた。 ・Translation: The government of that country was shocked by the sudden outbreak of war. ・Explanation: This sentence describes the surprise and impact of a war's beginning.

多くの人々は、開戦のニュースを聞いて悲しんだ。 ・Translation: Many people were saddened by the news of the war's commencement. ・Explanation: This sentence illustrates the emotional response to the beginning of a war.

The word 開戦かいせん is composed of two kanji: 開かい, which comes from the verb 開くひらく (to open), and 戦せん, which represents "war" or "battle". Therefore, the etymology literally translates to "opening war" or "starting war," directly indicating the commencement of military conflict.