The winter was approaching; and, instead of withdrawing his troops from the occupied provinces and marching them back into Syria, he resolved to keep them where they were, merely dividing them, on account of their numbers, among the various cities which he had taken, and making them go into winter quarters.

It was, no doubt, his intention to remain quiet during the two or three winter months, after which he would have resumed the war, and have endeavored to penetrate through Media into Parthia Proper, where he might expect his adversary to make his last stand.
But Phraates saw that the position of affairs was favorable for striking a blow before the spring came.

The dispersion of his enemy's troops deprived him of all advantage from the superiority of their numbers.
The circumstance of their being quartered in towns newly reduced, and unaccustomed to the rudeness and rapacity of soldiers and camp-followers, made it almost certain that complications would arise, and that it would not be long before in some places the Parthians, so lately declared to be oppressors, would be hailed as liberators.
Moreover, the Parthians were, probably, better able than their adversaries to endure the hardships and severities of a campaign in the cold season.

Parthia is a cold country, and the winters, both of the great plateau of Iran and of all the mountain tracts adjoining it, are severe.

The climate of Syria is far milder.