Organized marauders, a serious thorn in the back of regular armies

[Sanjuro]

Without aviation units, or sufficient fuel to mount a pursuit, perhaps an effective defence for an area against marauders is to copy Alfred the Great's defence of Wessex against the Vikings:
Starting from a defended core, each outlying township is fortified in turn (Alfred called them burhs) to the extent that each can defend themselves in the short term; reinforcements can be called from the neighbouring burhs while the defenders keep the marauders busy. This sort of static defence would be almost useless against a modern army, but against lightly armed marauders (who would probably be unwilling to take serious losses) it might work.