Simplify the following text for a Grade 7 native English-speaking person.
Original: When the First World War began in 1914 , Kellner was called up for active duty as a sergeant and deputy-officer in the Prinz Carl Infantry Regiment ( 4 .
Simplified: When the First World War began in 1914 , Kellner was called back to active duty . He was an officer substitute in the Prinz Carl Infantry Regiment ( 4 .