When the United States lost the Vietnam war, the Vietnamese showed another face of war:
With a big difference of power, the weaker country can win.
This is happening again in Afghanistan and in Iraq. In Afghanistan the United States, one of the richest countries in the world, fights with one of the poorest and even with this difference it isn't able to win the war.
So, what I'm asking is: can an invading country win the war if and only if the people of this invaded country are not willing to fight hard enough?