I believe human's capacity for imagination is much less than that needed to realistically conceptualize an alien invasion.  Whatever it might be, it sure won't be beatable by Will Smith or Red Dust.

That doesn't stop the brightest minds of our time from attempting to grasp what might happen, when it could happen, if it has happened and what we might do when whatever happens whenever.

'Science Fantastic' host Michio Kaku recently took on the subject for MSNBC.