If Itachi would have sided with the Uchiha clan, would the ninjas of the the Hidden Leaf, could they have truly stop the Uchiha Clan? [I mean, who could stop Itachi from within the Hidden Leaf?]
[Danzo's brainwashing is very effective! =D]
The Uchihas would have dieed as Danzo threatened, Itachi is not dumb to be bluffed like that, he went with the choice that would kept Sasuke alive. Shisui was said to have been stronger than Itachi and Danzo took his eyes, the strongest genjutsu user in the Uchiha clan at that time lost to Danzo. Konoha has Hiruzen, Danzo, 3 tomoes Kakashi, Guy, Anbu black opt that have ppl like Yamato, anbu root that have ppl like Fuu and Sai, The Hyuga clan, the nara clan, Ino's clan, Choji's clan, Kiba's clan, medical ninjas to heal the wounded then they have the jounins, chuninsand genins.
Plus the fact that not all Uchihas had Sharingans, it only awakened in a small few at that time unlike Madara's time which was constant war. Danzo only counted him, Hiruzen, the Hokage's anbu and anbu root and not the other clans, if they got the other clans to aid them then it's low diff. Hanzo was was a bit younger than when Oro fought him so he make have been a bit stronger back then.