Translation guide
The First World War (World War I) is typically referred to as 第一次世界大戦 in formal contexts, but in everyday conversation, 第一次大戦 or simply 大戦 may be used when the context is clear.
First World War
The full, formal term for World War I. Used in textbooks, news, and official documents.
The First World War began in 1914.
A slightly abbreviated form, still formal but common in historical writing.
The impact of the First World War was enormous.
Literally 'the Great War'. Can refer to WWI when context is clear, but may also refer to WWII or other large wars. Use with caution.
Ambiguous without context; usually refers to WWI in European history, but in Japan it might be unclear.
During the Great War, many soldiers lost their lives.
When talking casually about the First World War, often the full term is still used, but abbreviations or contextual references are possible.
WWI is 第一次世界大戦 (Daiichiji Sekai Taisen) and WWII is 第二次世界大戦 (Dainiji Sekai Taisen). The pattern is 第 + number + 次世界大戦. In casual contexts, they are often shortened to 第一次大戦 and 第二次大戦, or simply WWI/WWII when speaking with people familiar with English abbreviations.
World War II ended in 1945.
Japanese speakers often use the English abbreviation 'WWI' (ダブリュー・ダブリュー・ワン) in casual conversation, especially among younger people or in contexts where English loanwords are common. However, in formal writing or speech, the Japanese term is preferred.
Slightly shorter, acceptable in casual conversation among history buffs.
I watched a movie about the First World War.