First Send Them Back to the Stone Age, Then Make Their Home Into a Parking Lot
A smart world conqueror might put off invasion plans until nano-weaponry becomes more advanced. Nothing clears a space so well as nano-invaders capable of stripping a land cleaner than locust. Stealthy in, quick working, then dissolving into the landscape. Entire nations could be turned over, ready for radical makeover practically overnight!
The social democracies of Europe, North America, Japan, and Oceania have allowed humans to take a vacation from history for the past half century or more. This pax cannot possibly last much longer, particularly in the wake of the ongoing Obama devastation of the world economy.
Soon, the world populations will be thrust back into the hands of the warlords, the conquerors, the absolute destroyers of lands and peoples. Again. History coming to life, just when you thought it was obsolete.
Modern weapons research is a bit uneven. Most projects, perhaps, will not survive due to poor conceptualisation and a lack of robustness on the field of battle. But enough of the tools of "the new warfare" will succeed, to change the face of the planet a million times more than the hyped up global warming scare of Gore and Obama.