There is no guarantee that a war would have broken out. Hiruzen Sarutobi said he was going to try to talk it out. Fugaku was a reasonable guy as you saw from his death, so there is a chance things could have worked out. This is the reason that Danzo seemed displeased, and gave Itachi that ultimatum behind Hiruzen´s back. You can obviously tell that Danzo was manipulating, intimidating, lying and rushing him.
The leaf forgave the sand, and even before Sarutobi realized that it was Orochimaru, he STILL tried to solve the problem diplomatically DURING the take over, when they had already killed his right hand man! Everything resolved to the best without a mass genocide. Itachi had no faith in Sarutobi, relied on just himself, and paid the price.