I have just noticed the following situation in a great campaign:
I was controlling 3 client states when a faction declared war on me. One of them joined the war, the other two refused to do so because they did not like me very much. Now these two factions are independent but not in war with me. There even seems to exist a truce between us, so I can not attack them without losing diplomatic reputation although it would be my right to subjugate them again.
I am not sure but I think in previous versions refusing a call to arms meant war with your former overlord as a client state or satrapy.
The idea of client/patron relationships had significant implications for the later Roman Empire and even medieval society. As Rome expanded throughout the Republic and Empire, it took over smaller states which had its own customs and rules of law. The US officially has control over the foreign policy of Micronesia, Palau, and the Marshall Islands.
Rome 2 Client States At War
Rome Ii Client States
For Total War: Rome II on the PC, a GameFAQs message board topic titled 'Subjugation, and client states'. It was a hard fought victory for Rome, however; Hannibal Barca’s campaign in Italy, following his daring crossing of the Alps, nearly brought Rome to ruin. Catastrophic defeats at Trebia, Lake Trasimene and Cannae shattered any faith Rome’s allies had in them, and almost imploded the Republic.