Simplify the following text for a Grade 8 native English-speaking person.
Original: Cassius ' first office was as quaestor under Marcus Licinius Crassus in 53 BC , an office in which he proved himself to have a capable military mind .
Simplified: Cassius 's first office was as quaestor under Marcus Licinius Crassus in 53 BC , and he proved a good military leader .