You clearly can't interpret words on a page unless they serve your own purpose......Danzo says:
"Whether a war happens or not!!"
That^ means EVEN IF Hiruzen manages to stop the coup and the Uchiha, Danzo, by his own judgement, has deemed the Uchiha Clan as traitors and that they are fated for annihilation.
Keep in mind, that is what Danzo wants Itachi to think, but that is not what Hiruzen, the Hokage, will decide.
Danzo knows Hiruzen will go for the peaceful solution, and at this point, Danzo will say whatever it takes to put Itachi at the point of no return in his mind, and carry out the annihilation himself, by using Sasuke's life as a bribe.
Also keep in mind, Itachi wasnt even a teenager then, and he regrets making that decision, realising it as a mistake. Itachi was manipulated.