So, I went with the White Huns campaign, first turns:
-sacked& razed Merv
-sacked Dahistan, then Parthia retook it, I had the option to subjugate them and did that
The campaign went tits up shortly later because I didn't know how to manage horde economics.
Tried it today, same start:
-sacked and razed Merv
-sacked Dahistan, can't subjugate Parthia
One of the sub-objectives is to have 3 clients, so I'd like to do that. What am I missing?