So I head canon that Z was able to conqueror the Earth with the help of 3rd Society tech (Husk, Dreadnaughts), but also using Indoctrination to turn resistance personnel against each other. At first it was extremely effective, the indoctrination of key personnel would bring about confusion and distrust within the resistance, then she'd swoop in with the Husk and Society dreadnoughts and clear house.
In my mind Z's stronghold was in Europe which didn't actually fight rather surrendered before the war started off. They gave seemingly endless resources to fund this war without the rest of the world knowing. There was a scientist living in the United Kingdom that actually figured out the Societies "Alien Algorithm" which if reconfigured could turn off the IFF for all Society technology (They'd see each other as threats).
This information got to the Resistance and the "Intel" teams figured it out, launching there attacks from Asia and Northern Africa (To almost Pin Z in Europe), but then Thee Champion returned