The reason is that the dollar's role as reserve currency is at stake. If the Federal Reserve has to monetize the federal deficit, the world will turn its back on a rapidly depreciating dollar. The minute the dollar loses the reserve currency role, the U.S. can no longer pay its bills in its own currency, and its days as a superpower come to a sudden end. Wars can't then be financed, and Washington's pursuit of world hegemony will hit a brick wall.
http://www.opednews.com/articles/1/Is-The-Recovery-Real-by-paul-craig-roberts-100302-276.html
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