Crni Vuk, the point is, that a resistance movement (or terrorist group
) does not engage the same way as an army. They can't beat an Airattack or an onrushing tankbattalion, so they don't even try. The goal is to make an occupation too costly for the invader, not beat him outright.
In an Invasion setting in America I think resistance would be an important factor. As soon the army would realize that it can't beat the invader, they would start to hide weapons away for guerrilla wars. Man portable antiair and -tank weapons for a preference. In afgahnistan the US stingers really gave the Russians food for thought. Not that you can beat an army with such weapons, but it makes it costly. Commanders will think twice before deploying an expensive gunship into a small skirmish if they run the risk of loosing it. So in small engagements, traps and ambushes the groudhogs would be left to fend for themselves. This is terrible for morale. So is having to worry about poisoned food or a slit throat while sleeping. Guerilla is about constant pressure, the perceived threat is larger by far than the actual manpower or weapons might suggest.
As was said, the size of the country, the amount of wilderness to hide in and the amount of small arms supports guerrilla warfare. Sure, a lot would be caught, pressed into surrender or starved of supplies, but I guess there would remain enough to make life hard on any occupation force. And as history has told on numerous examples, the longer that kind of war goes on, the better the resistance gets while the invader gets tired and demoralized.
Only works if the invader has to justify the cost (monetary and blood) at home. If it is a migration and they have no place to retreat to, things could be different.
Edith: no, the germans and japs were tired of war. The germans more than the japs, but the Japanese are pretty obedient, so when the emperor said to stop, they stopped. The germans welcomed the allied occupation because it was better than the war, and the japs were resigned.
But look at what the French and the jugoslavian (well, not jugoslavia at that time, but you know what I mean) resistances did during WW2. Those are examples of what happens when the people want to keep on fighting. Germany and Japan of what happens, if they don’t. Fun thing is that both ways worked out pretty good in hindsight.