、。
The following year, the First World War began.
Literal
Next-year, First [attributive-の] World War [subject-が] began.
翌年 ('the following year,' read よくねん) is a formal time-shifter, literally 'next-following year.' Used in historical narratives to shift to the year after the reference point. 第一次 ('number one / the first') + 世界大戦 ('world war') = standard Japanese term for WWI.