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Financial markets have long treated U.S. bonds as the ultimate safe asset; the assumption that America will always honor its debts is the bedrock on which the world financial system rests. In particular, Treasury bills — short-term U.S. bonds — are what investors demand when they want absolutely solid collateral against loans. Treasury bills are so essential for this role that in times of severe stress they sometimes pay slightly negative interest rates — that is, they’re treated as being better than cash.

Now suppose it became clear that U.S. bonds weren’t safe, that America couldn’t be counted on to honor its debts after all. Suddenly, the whole system would be disrupted.

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– http://www.nytimes.com/2013/09/30/opinion/krugman-rebels-without-a-clue.html?ref=todayspaper