Well WWZ takes place over a 10 year time span. The "after action" report that Max Brooks is supposedly writing for the UN is shortly after the end of major hostilites more than a decade afte the Great Panic. I believe in the book it was not long after the start of the Great Panic and the collapse of the major cities that the US and most other functioning military authories decided to follow the Redekker Plan and simply fall back to fortified positions and regroup/rebuild. It was several years after the Panic that the UN started to coordinate the global offensive. Season 3 of the Walking Dead takes place about 1 year after their version of the Great Panic. Remeber in Season 2 we did see some flashbacks to the early military attempts to keep the Zs at bay. Remember, Atlanta was firebombed (thoug, oddly it showed no signs of it while they were in the city). Even assuming that TWD followed WWZ's timeliine, we would still be a couple of years away from any organized military offensive. Plus, even in the book, there were communities and people that went years without seeing ANY efforts from armed forces.