Ignorance of history can lose wars and sink nations.
Without a good grounding in history people can start to believe in their own society's infallibility. They may assume that good times are inevitable and that each generation will always fare better than its predecessors. Listen carefully to them and you may even hear confidence that some odd form of magic will kick in at a crucial moment and erase the effects of multiple blunders.
Study history and you'll know better. You'll know that although catastrophes can happen, the usual decline is slowly achieved. A family can sip coffee, read the newspaper, listen to the radio, and pleasantly pass the time while their civilization starts to circle the drain.
People worry about the math scores of high school students. I worry more about the quality and quantity of the history they are learning.