There were rumours that Crassus was involved in the Catiline conspiracy (63-62), and Plutarch (Crassus 13:3) says that Cicero specifically stated after their deaths that Crassus and Julius Caesar were both involved in the conspiracy. Unfortunately, that speech has not survived, so we don't know what exactly Cicero said.
Julius Caesar persuaded Pompey and Crassus to settle their differences, and the three of them together formed the informal association which is often referred to as the first triumvirate (although, unlike Octavian, Antony, and Lepidus, they were never officially appointed as a triumvirate) (60).
In elections disturbed by serious rioting, Pompey and Crassus were elected consuls again for 55. In the distribution of provinces, Crassus was appointed to govern Syria. It was widely known that he intended to use Syria as a base for operations against Parthia, something which aroused considerable opposition since Parthia had never done the Romans any harm. Ateius, one of the tribunes, tried to stop Crassus from leaving Rome. When the other tribunes did not allow Ateius to detain Crassus, he called down a formal curse on Crassus as he left the city (54).
When Crassus crossed the Euphrates into Mesopotamia, many cities with Greek populations came over to his side. He garrisoned them and then withdrew back to Syria for the winter, where he waited for his son, who had been serving with Julius Caesar in Gaul, to join him. Rather than spending the time training his troops, Crassus pretended that he was going to levy troops from the local rulers so that they would bribe him not to.
The Parthians attacked the garrisons Crassus had installed the previous year, and stories came back of their devastating archery and impenetrable armour. The Parthians had perfected the art of shooting arrows backwards off a galloping horse, and this is the origin of the English expression, Parthian shot. Although his men were dismayed by these stories, Crassus left his winter quarters for Mesopotamia (53), encouraged by the support of King Artabazes (otherwise known as Artavasdes) of Armenia, who brought 6000 horsemen, and promised a further 10,000 horsemen and 30,000 foot soldiers. Artabazes tried to persuade Crassus to invade Parthia via Armenia, where he could provision the army, but Crassus insisted on going through Mesopotamia. His own army consisted of seven legions, plus nearly 4000 cavalry and about the same number of light-armed troops.
To start with he proceeded along the Euphrates, towards Seleucia, but he allowed himself to be persuaded by an Arab called Ariamnes or Abgarus, who was secretly working for the Parthians, to cut across country to attack the Parthians under Surena. (Surena was one of the most powerful men in Parthia: his family had the hereditary right to crown the kings, and he himself had helped restore the reigning Parthian king, Hyrodes or Orodes, to his throne.) Meanwhile, Hyrodes had invaded Armenia and was fighting Artabazes.
Ariamnes led Crassus into the desert, where Crassus received pleas from Artabazes for him to come and help fight off the Parthians there, or at least keep to mountainous areas where the Parthian cavalry would be useless. Crassus took no notice but continued to follow Ariamnes.
