public treasury since personal wealth was not what Crassus most lacked. Most modern historians tend to view insatiable greed, envy of Pompey's military exploits and rivalry as his motivations since his 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. Plutarch noted that Caesar wrote to Crassus from Gaul and endorsed the plan to invade Parthia, an indication that he regarded Crassus's military campaign as complementary and not merely rivalrous to his own. 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. Cicero, however, suggested an additional factor: the ambitions of Marcus's son, the talented Publius Crassus, who had commanded successful campaigns in Gaul under Caesar. Upon his return to Rome as a highly decorated officer, Publius took steps to establish his own political career. Roman sources view the Battle of Carrhae not only as a calamity for Rome and a disgrace for Marcus Crassus but also as a tragedy that cut short Publius Crassus's promising career. Some Romans objected to the war against Parthia. Cicero calls it a war nulla causa ("with no justification") on the grounds that Parthia had a treaty with Rome. The tribune Ateius Capito put up strenuous opposition and infamously conducted a public ritual of execration as Crassus prepared to depart. Despite protests and dire omens, Marcus Crassus left Rome on November 14, 55 BC. Publius Crassus joined him in Syria during the winter of 54–53 BC and brought with him the thousand Celtic cavalry troopers from Gaul who remained loyal to their young leader until their death. Invasion of Parthia Crassus arrived in Syria in late 55 BC and immediately set about using his immense wealth to raise an army. According to Plutarch, he assembled a force of seven legions for a total of about 28,000 to 35,000 heavy infantry. He also had about 4,000 light infantry, and 4,000 cavalry, including the 1000-strong Gallic cavalry that Publius had brought with him. With the aid of Hellenic settlements in