diplomatic_stance
Availability: campaign
Usage: diplomatic_stance <faction_a> <faction_b> <allied/neutral/war>:
Set the diplomatic stance between the two factions
So the usage in a script would look like this:

Code:
     console_command diplomatic_stance romans_brutii parthia neutral
     console_command diplomatic_stance romans_julii parthia neutral
     console_command diplomatic_stance romans_scipii parthia neutral
     console_command diplomatic_stance romans_senate parthia neutral
That makes all the roman factions "Neutral" toward Parthia, whether they were are war or allied. Note that only allied, neutral and war are available - there does not appear to be a way to force protectorates (I only mention this since that's what people usually seem to be looking for).