By the mid 2nd Century B.C.E. the Seleucid empire was beginning to crumbled and a general chaos began to errupt. Various districts bid for independence; the most priminent break away kingdoms was that of the Arsacids. In the east, under the leadership of Mithradates I, they wrested regions of Bactria away and in the west Media was subdued. Also, by the beginning of the first century B.C.E. the Romans began making their bids for regions on Anatolia. In the east Scythian tribes began to cleave territories away. It was not until Mithradates II and Phraates III that the empire was restored to a reasonable order. This was the Parthian Empire. However, even as it maintained the visage of an empire, there were significant structural differences between it and the Achaemenid dynasty of a few centuries before - primarily, it was much weaker, as can be seen its lack of effort in assimilating the various smaller states within the empire into a single fuctional unit, as had been done by the Achaemenids.

What are of primary interest here are two points:

1) The donning of Parthian kings of composite names including the root "Mithra" exemplified the significant stature that Mithra bore in the religion.

This was not only a phenomena in the east within the Parthian dynasties, but also the West in kingdoms such as that of Pontus, which was established by Mithradates I Ctistes who, during the the wars of the Diadochi (the battles of the Greek generals following Alexander's death) forged his Pontic Kingdom. The region of the Pontic Empire eventually stretched from Cappadocia, its northeastern Anatolian home to include much of Anatolia, the Crimean region, and parts of what is now southern Ukraine under the reign of Mithradates IV Eupator. Skirmishes between the Pontic kingdom and Greece marked different points in its timeline, until it was eventually conquered by Pompey in 66 B.C.E., marking the end of one of Rome's greatest adversaries.

2) The Romans in their drives into Asia Minor, for the first time came significantly in contact with Mithraism.

One of those contacts is recorded by Plutarch, who explained how during the Mithradatic wars between Rome and the Pontic kingdom, Cilician pirates hired themselves out to the king; some were noted to perform strange rite atop mount Olympus in praise of the god Mithra which, and Plutarch admits, were religious mysteries later preserved by the Romans. This in itself creates a hardy link between the later Romanized Mithraism and the Mithraism begotten from Mazdaism.

At any rate, as can be clearly seen from the nature of Mithra himself, or Mithras as the Romans called him, he had a great appeal towards the military from his own spiritually militarisitic nature. In the Avesta he was understood as the god in whom invocation would bring victory in battle, and this without doubt, would be appealing to any soldier weighted by the mortal vicissitudes of war. Moreover, by this time Mithraism had reached the status of a mystery cult. Many Romans had become disillusioned by the flagrant hypocrisy and dun coloured nature of their religion and found in Mithraism a spark of eastern mysticism and power that was far more attractive. Also, Roman military policy dictated that Roman legionaries not serve in a district that they were born or raised in, and were often stationed in far regions of the empire - also, Roman armies during the first century B.C.E. and the centuries during the Christian era were constantly sent to combat various foes, be it Germanic tribesman in the region of the Danube, or to quell disruption in Asia Minor, and as such often found their officers and legions intermingling. For these reasons Mithraism quickly spread through the Roman occupation armies in Asia Minor, primarily through proselytizing legionaries.