Meanwhile, however, a revolution had occurred in the Parthian empire; the grandees of the kingdom, with the young, bold, and talented grand vizier at their head, had overthrown king Mithradates and placed his brother Orodes on the throne.
Mithradates therefore made common cause with the Romans and resorted to the camp of Gabinius.

Everything promised the best results to the enterprise of the Roman governor, when he unexpectedly received orders to conduct the king of Egypt back by force of arms to Alexandria.( 3) He was obliged to obey; but, in the expectation of soon coming back, he induced the dethroned Parthian prince who solicited aid from him to commence the war in the meanwhile at his own hand.

Mithradates did so; and Seleucia and Babylon declared for him; but the vizier captured Seleucia by assault, having been in person the first to mount the battlements, and in Babylon Mithradates himself was forced by famine to surrender, whereupon he was by his brother's orders put to death.
His death was a palpable loss to the Romans; but it by no means put an end to the ferment in the Parthian empire, and the Armenian war continued.

Gabinius, after ending the Egyptian campaign, was just on the eve of turning to account the still favourable opportunity and resuming the interrupted Parthian war, when Crassus arrived in Syria and along with the command took up also the plans of his predecessor.

Full of high-flown hopes he estimated the difficulties of the march as slight, and the power of resistance in the armies of the enemy as yet slighter; he not only spoke confidently of the subjugation of the Parthians, but was already in imagination the conqueror of the kingdoms of Bactria and India.
Plan of the Campaign The new Alexander, however, was in no haste.