When Viola thinks she sees Todd getting hit by one boomerang, actually the Mayor's horse, she launches a missile that kills all the Spackle archers and blocks the way up and down the hillside, separating the two armies.
When Viola thinks she sees Todd getting hit by one boomerang, though it was actually the Mayor's horse, she launches the missile, which causes a catastrophic explosion. This kills all the Spackle archers and blocks the way up and down the hillside, separating the two armies.