Though the author of the saying, "Those who ignore history are bound to repeat it" is disputed, what we are seeing today (August 15, 2021) is a perfect example of what can happen when we do ignore history!
My book on the Vietnam War, "Warriors and Fools: How America's Leaders Lost the Vietnam War and Why it Still Matters" in its last third of the narrative argues that there were important lessons learned from the Vietnam War that we are ignoring today.
Moreover, the lack of adherence to or use of those lessons has lead and is leading to the misuse of our military in the world today. The vindication is what has happened in Afghanistan.