The US Oil Dollar
At the end of WWI, our currency shifted from the "gold standard" over to what is now called the petro-dollar, or the oil dollar. So as the price of oil rises, our dollar value drops.
For decades, OPEC's currency for all exported oil was the good old greenback. And today, 50% of the strength of the United States currency is dependent on cheap, plentiful oil. While the Euro gains in price, and the dollar drops fears have begun to develop that OPEC could successfully shift its standard currency from the U.S. dollar to the Euro, as Iraq and Iran have tried to do.
The value of the dollar would plummet by roughly 50% and we would find ourselves in an economic state far worse than the Great Depression. The terrifying part is, without our military presence in the Middle East, it would probably happen.