A currency war is where competing national economies devalue their currencies to spur exports and to pay back loans in currency that is worth less than at the time the loan is made.
http://en.wikipedia.org/wiki/Currency_war
Last currency war was in 72, the Nixon shock, leading to a period of continuous recessions that lasted until the Reagan years. The various economic bubbles created in the time since the Reagan era represent failed attempts to resolve the underlying economic issue in accordance with neoclassical economic thought and have lead to a debt bubble of immense proportion, it's only a matter of time until that collapses. Already blowback from what we have to do to maintain the bubble is causing major geopolitical instability. A hot war is imminent.
Before that was in the period from the 20s leading to WWII, a conflict which was caused by the extreme political reactions of Germany and Japan to the manipulation of the global economy, and therefore the very fabric of those societies. We suffered through the Great Depression but the damage done to those societies was incalculable as was the damage resulting from the inevitable social result, WWII.
Currency wars are among the most destructive social forces known and we are entering one right now. The BRIC nations are importing gold in mass quantities to staves off attacks on their currency by USD and banking in the BRIC nations are challenging the dominance of USD:
http://www.zerohedge.com/news/brics-ban ... -dominance
Important book:
http://www.amazon.com/Currency-Wars-Mak ... 1591845564
Massive reallocation of human capital is imminent.