[Dreamer128]

I think history teaches that a country can win every major battle and still lose the war. Good ol' Pyrrhus was one of the first to figure that out. Still, the reasons he got kicked out of the Latin states aren't even remotely the same as the problems that democratic countries face when waging war abroad. The main issue is public opinion. The insurgents know that if enough bodybags get shown on American national television, the US will leave at one point or the other. They just have to keep their heads low until it happens. They knew in Vietnam as well. It is said that the only purpose of the Tet Offensive was to influence the American elections, which meant they sacrificed thousands of men to influence the American public opinion. The insurgents, on the other hand, do not have to get re-elected and have access to large numbers of fanatics who'd just love to throw themselves at impossible odds.
I've also been told that the Pentagon got rid of its 'nation building' department in the 90s. Apparently, they assumed that any future war would be fought against China. Now, I don't know to what this degree this information is correct, but a might explain a few things about the current state of Iraq and Afghanistan.