Second, as I've explained, this scenario with the gnolls would be something that would already be "in motion". This would happen over sessions of play if the players chose not to interact with those elements of play. It could be very easily summarized like this:
- Gnolls are raiding caravans along the western road
- Emboldened, the gnolls have begun to attack homesteads and farms not far from town
- The gnolls use their pillaged funds to hire a couple of giant mercenaries to bolster their force
- The gnolls attack the town
As a GM, you'd track these events and mark the progression from one step to the next, provided that the players don't get involved in some way. If the players do get involved, then you'd determine the impact that involvement would have on this track of events.