Greed is often regarded by the ancient sources, particularly his biographer Plutarch, as his major character fault and also his motive for going to war.[8] Historian of Rome Erich Gruen believed that Crassus's purpose was to enrich the public treasury, since personal wealth was not what Crassus himself most lacked.[9] Most modern historians tend to view insatiable greed, envy of Pompey's military exploits, and rivalry as his motivation, since Crassus’s long-faded military reputation had always been inferior to that of Pompeius – and after five years of war in Gaul, to that of Caesar. His major military achievements had been the defeat of Spartacus in 71 BC and his victory at the Battle of the Colline Gate for Sulla a decade earlier.[10] Plutarch notes that Caesar wrote to Crassus from Gaul, endorsing the plan to invade Parthia — an indication that he regarded Crassus's military campaign as complementary and not merely rivalrous to his own.[11] Another factor in Crassus's decision to invade Parthia was the expected ease of the campaign. The Roman legions had easily crushed the numerically superior armies of other eastern powers such as Pontus and Armenia, and Crassus expected Parthia to be an easy target.[12]