We know that contacts between the "Hosts" and government authorities in the world had been established before the Arrival, scheduled
perhaps back to the era of the Apollo missions on the Moon (what if the presence of Man on the Moon had triggered the count-down?).
Why did Humanity not try to resist to alien's invasion?
Either, because human weapons are no match with aliens' technology and they had no other choice than blindly complying with all their requirements,
or they had interest in collaborating with aliens for a reason(perhaps they were promised privileged positions in the New World Order).
Had the aliens' sole goal been to take Earth for themselves, their advanced technology would have allowed them to easily get rid of Humanity.
They do not seem to be interested in the Earth natural resources, such as water or minerals, or human or animal resources for food, either.
What they seem to need is human beings. What for?
Creating human-alien hybrids to save their dying species from extinction?
Enslaving humans to use them as a servile workforce or an army?
Using them as laboratory rats for some dark experiments?
Saving Humanity from extinction resulting from a scheduled worldwide disaster by selecting humans to be kept alive in pods on the Moon to repopulate the After Earth?
What do you think their motivations might be?