Crassus arrived in Syria in late 55 BC and immediately set about using his immense wealth to raise an army.
He assembled a force of seven legions (for a total of about 28,000–35,000 heavy infantry).
In addition he had about 4,000 light infantry, and 4,000 cavalry, including the 1,000 Gallic cavalry Publius had brought with him.
With the aid of Hellenic settlements in Syria and support of about 6,000 cavalry from Artavasdes, the Armenian king, Crassus marched on Parthia.
Artavasdes advised him to take a route through Armenia to avoid the desert and offered him reinforcements of a further 10,000 cavalry and 30,000 infantry.
Crassus refused the offer and decided to take the direct route through Mesopotamia, and capture the great cities in the region.
In response, the Parthian king Orodes II divided his army, taking most of the soldiers—mainly foot archers with a small amount of cavalry—to punish the Armenians himself, and sending the rest of his forces—9,000 horse archers and 1,000 cataphracts under the command of spahbod Surena—to scout out and harass Crassus' army.
 Orodes did not anticipate that Surena's force, outnumbered by almost five to one, would be able to defeat Crassus, and merely wanted to delay him.

Crassus received directions from the Osroene chieftain Ariamnes, who had previously assisted Pompey in his eastern campaigns.
Crassus trusted Ariamnes, but Ariamnes was in the pay of the Parthians.
He urged Crassus to attack at once, falsely stating that the Parthians were weak and disorganized.
He then led Crassus' army into the most desolate part of the desert, far from any water.
Crassus then received a message from Artavasdes, claiming that the main Parthian army was in Armenia and begging him for help.
Crassus ignored the message and continued his advance into Mesopotamia.
He encountered Surena's army near the town of Carrhae.
Crassus arrived in Syria in late 55 BC and immediately set about using his immense wealth to raise an army. He assembled a force of seven legions (for a total of about 28,000–35,000 heavy infantry). In addition he had about 4,000 light infantry, and 4,000 cavalry, including the 1,000 Gallic cavalry Publius had brought with him. With the aid of Hellenic settlements in Syria and support of about 6,000 cavalry from Artavasdes, the Armenian king, Crassus marched on Parthia. Artavasdes advised him to take a route through Armenia to avoid the desert and offered him reinforcements of a further 10,000 cavalry and 30,000 infantry. Crassus refused the offer and decided to take the direct route through Mesopotamia, and capture the great cities in the region. In response, the Parthian king Orodes II divided his army, taking most of the soldiers—mainly foot archers with a small amount of cavalry—to punish the Armenians himself, and sending the rest of his forces—9,000 horse archers and 1,000 cataphracts under the command of spahbod Surena—to scout out and harass Crassus' army.  Orodes did not anticipate that Surena's force, outnumbered by almost five to one, would be able to defeat Crassus, and merely wanted to delay him.
Crassus received directions from the Osroene chieftain Ariamnes, who had previously assisted Pompey in his eastern campaigns. Crassus trusted Ariamnes, but Ariamnes was in the pay of the Parthians. He urged Crassus to attack at once, falsely stating that the Parthians were weak and disorganized. He then led Crassus' army into the most desolate part of the desert, far from any water. Crassus then received a message from Artavasdes, claiming that the main Parthian army was in Armenia and begging him for help. Crassus ignored the message and continued his advance into Mesopotamia. He encountered Surena's army near the town of Carrhae.
Battle of Carrhae