You may be right however a quick scan of the forums showed this..
(Previously written by "Mightyoates")

"As noted, you need the "chance to power" event (8 or more popularity with the people). Or, you could get the senate to hate you (2 or less popularity with senate). They will ask your leader to commit suicide.

Unfortunately there is no other way to start the war. You have to wait for these events. The game stops you from commiting any acts of war on other Romans before that."

So you need the chance to power even first sorry...