Impact was tied principally to the number of actions the Syndicate could generate.
There is no
one factor that caused the Syndicate defeat. However, I would say that most of the elements that led to the defeat either arose from, or became involved in, the trio of Duaba, Lironh, and especially Celos.
Celos was a diversionary attack from Duaba that went disastrously right for the Syndicate. They were in no position to hold territory like that. Even so, they nearly finagled outright victory by bringing in the Cardassians. In the end, however, the arrival of the Aeroknights prevented them from retaining sufficient control of the planet to rouse the Cardassians to defend them, and they ended up getting effectively sold down the river. So in many ways, I suppose the
Enterprise succeeding in their mission to hijack the Cardassian comms buoy was the closest thing to a single turning point, since that enabled the Treaty of Celos to be negotiated favourably.