>I don't know anything about the Thirty Years war, but I doubt that there's any major recent act of stupidity that was caused by excessive cleaving to (or knowledge of) historical experience.
Let me introduce you to Balkans war(s) of the 1990s. People who had been living side-by-side peacefully for decades decided to rehash hundreds of years of historical conflicts which to outsiders looked insane.
>I suspect we'll keep on looking back at the great depression until it ceases to seem salient.
The Great Depression is no longer relevant since we don't live in a world on the gold standard. The factors that drove the economy into the ground in the 1930s (massive deflation and bank failures) are not going to happen again. Deflation because countries can print as much cash as they like (Greece excepted), while in regards bank failures we now know banks (and bankers) are too big to fail.
>Oh, and the Crusades explain quite a bit about the state of the middle east.
Nothing about the middle east today is the result of the Crusades except some misguided people thinking the Crusades are relevant.