Answer: When these tragedies first happen it has a negative outcome. Overtime generations learn from these hard times and figure it ways to not have it happen again. For example, because of 9/11 all airports have more security checks and police to make sure nobody tried to hijack the planes again. For the holocaust, the Berlin Wall was created, so America and Russia could watch both sides and keep the western fascists from entering East Germany. The world doesn’t want to repeat history, so by learning from these outcomes we can prepare future generations in case, somehow, this happened again. Hope that helps!
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