Nobody was expecting the Vlain to come to earth.
Nobody knew what to expect after the arrival, either. What would they look like? How would they react to humans? What was their purpose? And of course, the most important question to our paranoid heads - if the time came for war, would they be stronger?
Humanity made a two-part mistake. They came prepared for a fight against a tangible enemy, and without any attempt at negotiation. Who said missiles and bullets would work against them? and who made the decision that they were enemies in the first place?
The way the Vlain worked - nobody expected it. And nobody really knows how they work now, either.
But I do.
Humanity had already done something to itself. We'd already pushed ourselves up to the edge of the cliff - the Vlain just gave us the last push. We planted the bomb, and they primed it.
Something clicked on the day of Arrival. An ever so insignificant, subtle little change in the universe. Unnoticeable. Yet that's all it took, like the last tremor that set off the avalanche, the missing support that brought the whole building down.
Then the world went to hell.
or changed for the better.
Depends on how you look at it.