None. Those people died completely in vain.

Had lessons been learned they might have included:

Don't fight someone who wants it more than you do on their own territory without overwhelming force. Corollary: Don't fight a self-perpetuating population willing to die for their cause when you aren't willing to die for yours. They will grind you down.

Don't plan on winning a war unless you are willing to take AND HOLD real estate with a continual presence. Corollary: Don't expect to win "hearts and minds" unless YOU can provide day-to-day security for the population at large, either yourself or through a native infrastructure which is subservient to you. People want to be safe and left to raise their families, and will support those who provide that level of security.

Don't fight a guerilla war or insurgency without regarding your mission as "hearts and minds" first, placing that strategy BEFORE your immediate tactical objective when the long-term damage will be greater than the short-term advantage. Corollary: The soldiers primary mission is not necessarily to survive any engagement regardless of cost to the civilian population. If the mission were to come home alive at all costs, you wouldn't be over there in the first place.

Don't believe your own propaganda. You are not as pure and good as you believe, and the enemy is not as bad and evil as you are told. Your enemy has good reasons for what he does, is smart, knows the land and language and has his back covered.

It's sad the US is going through the same damned thing, again, without learning. It's clear that when "Captain America" was discontinued that was one less book for the CinC to read each month.

Arf