[robinzfc]

Or maybe they showed up 200 years ago and thought: this is a very nice planet, except it's a bit too cold. What can we (compel the natives to) do to make it some 6 degrees warmer on average so that it is perfect for us? And the rest is history.

[jandrese]

This is kind of the plot of Pacific Rim. The aliens originally invaded in prehistoric times, but then the Earth cooled and they had to retreat to their dimension. Humans then warmed it back up so they could restart the Kaiju invasion plans.