It wouldn't be the smartest thing to destroy a quantity of our arable land that they want too to use for it's resources of growing crops and raising livestock, besides being there for future useable development.
Large populations would still be left to rise up in anger and our weapons would further be turned on them in return.
Disabling our counter and preemptive strike capabilities with quick and decisive key strikes to do as little damage to potential gains while taking out as much as possible to weaken us is the smartest way. Killing as many of us as useful is another and weakening our ability to fight back too. That's what's best in the long run, not making all the arable land glow for years to come while leaving the population at large open to work against them for even a second more.
Even destroying major cities will destroy industrial gains and useful resources. Sneaking in and flooding the country with an engineered virus that you have the antivirus for to slowly but surely kill off the population while leaving everything else untouched is the best choice especially if you can do under the table and deny any involvement till your ready to strike in force with your forces and tachnology at your chosen moment. (even though sinister, what's new about the human race though)
If the virus could sweep through population and long distances quickly killing as it goes while being blocked off by quarantine by the rest of the world only sending aid and looking on they'd have a much easier time of it with the actual power to stand up to anyone else after it's done.
Combined military forces of China, Russia, North Korea, Iran and other Muslim forces, plus possible others like Argentina, Cuban and any small joiners could give the world a run for it's money if played smart.
As far as I'm concerned it's naive to think someone isn't thinking it and looking towards it as a possibility right now as we see military strength around the world continue to built and China's at a large rate.
(Damn I need a new gas mask)