Well that's because the US hasn't been invaded since the Revolution.
You can't use Guerilla Warfare as an invading army, because it is a purely defensive strategy. It's a case of hiding and doing hit and run tactics.
You can't do that if your aim is to take land away from someone else, particularly if it's not your native land that you know how to use.
Homefield advantage, it gives you the ability to overwhelm and wear away at an enemy slowly. It seems generally good to be on the defensive.
The only way to fight against guerillas is to fight dirty, kill every civilian and burn everything to the ground. It's considered immoral and costly, but it's effective.
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