The other week I was watching the movie Independence Day which was on I would suppose because it was Fourth of July. I was thinking about the strategy that the aliens used in order to take over the Earth. From the alien perspective, they would not know all the details of human civilization and details of complex inter-dependencies in human society, but I'm sure they would at least be aware of network effects and know that large concentrations of human society i.e cities would appear to be "mega-nodes" as far as human-to-human connections are concerned. Thus, not knowing any of the details of human civilization, it could be surmised that the simplest strategy would be to "degrade" their target's network as fast as possible by destroying the mega-nodes first. Thus in the movie, they destroyed the 30 or so largest cities in the world at the same time and then systematically destroyed the smaller cities. It would pretty much be a 'decapitation strike' against the humans which would severely degrade Earth's ability to fight back effectively. Of course, in the movie, the humans fight back and win as Hollywood generally likes a happy ending, at least for mass-appeal faire. In a more reality-based circumstance, the situation would be more dire and less hopeful.
With the decimation of society comes the loss of many of the expert 'nodes' and logistic chains to maintain it, thus any destruction of the population and infrastructure would set the human civilization back a few centuries, i.e back to ~ 1800 when most workers were agricultural and the industrial revolution was just starting to get underway. Since (hopefully) there would still be remnants and pockets of technical expertise left, I believe the society would bounce back faster than the original building out of the 1800s but more like rebuilding of Europe and Japan after world war 2 except not as fast as that since the loss would be be much greater than even those conflicts produced.
