The US is trillions of dollars in debt. The gold holdings in Fort Knox are a few hundred billion at most. You can't prop the dollar up with gold if there's a major economic boo-boo - the value just isn't there to offset such massive deficiencies. When they tried exchanging dollars for gold under the BW system they started to run out of gold.

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If anything's propping up the American Dollar, I suspect a far more likely culprit would be that OPEC primarily uses dollars - creating a demand for dollars in the international exchange markets.