It is good that someone is asking questions like these
So say Satrap A attacks Satrap B, and Satrap C is good friends with Satrap B, he can join the civil war himself, and throw all his eggs in the basket. but he cant send a phalanx to help?
If Satrap C wants to help Satrap B then he must make a Declaration of War against Satrap A to be able to give aid to Satrap B. After he makes the declaration he can send armies to help Satrap B.

If you are fighting against Basileus then things will be whole lot different because if the Chacellor is a good friend of the Basileus then Chancellor can end all building/recruitment in your Satrap.

Also we have our sub-forum in Throne room.

You can find it here