Answer :
The mechanized death of modern war.
Explanation:
The US should have known out of everyone that was in the conflict, the scale of modern war considering they had just gotten out of a civil war some 50 before. This is one thing the entire world should have looked at. The American Civil war was more deadly a conflict than the world had seen up to that point. The Americans especially should have realized that they dont want to be in a modern conflict again considering it was so costly before.