Yes, they could.
They never would have suspected Itachi, their prized gem and pride of the clan, to kill them. Itachi killed them quickly, quietly, and swiftly.
Had the entire clan gotten together and prepared for their assault on the village it would have been mayhem.
They'd weaken the village from the inside out just like the Sand shinobi did. How? A surprise attack.
If the Sand could cause so much destruction, then I think a clan comprised of natural geniuses could too.
After Konoha got weakened other villages would see that as the perfect opportunity to invade. The other villages would all want this boost in territorial power. They'd all jump in and a war would begin.
Tobi explained this to Sasuke