Nazi reprisal tactics were - slightly - less brutal in countries they perceived as Nordic or Aryan. Hitler also seemed to have some shred of respect for the Brits. It makes for fun alternate history, though. So I'll have to get that book. But not every enemy is prepared to use Nazi or Soviet style tactics.

The Viet Cong weren't rendered ineffective until the Tet Offensive (which may have been North Vietnam's intent) when they tried to engage us in quasi-conventional combat, and the Vietnamese held up to the Japanese, too, with minimal outside aid. The Afghani's also have a pretty good track record against occupying powers, with and without substantial outside support.

Fear of popular resistance entered into the equations of Japanese war planners, who decided not to plan for invading the US. Along with much more compelling reasons, they rightly feared a "rifle behind every blade of grass" in civilian hands (quote usually mis-attributed to ADM Isoroku Yamamoto, IJN). Likewise, the Germans wisely chose not to mess with the motivated and well-armed Swiss.

Suppression of indigenous popular resistance is also very resource intensive; a handful of clever resisters can tie up a disproportionately large number of occupiers for rather a long time. The outcomes depend on the methods, the players, their objectives and the particular circumstances, but no military leader really wants to engage in asymmetrical warfare with an active popular resistance.