In T R Reid’s book “The United States of Europe” he points to a growing threat from the Euro which is ignored by the US at its peril

He points out that the inflow of US dollars from foreign purchase of Treasury bonds, has historically eased the pain of (or funded, if you like) the huge trade deficit ($700m 2005). In 2004, Euro denominated bonds rose to $4 trillion, almost equal to the amount invested in dollar-denominated bonds. In a 2005 survey, two-thirds of the world’s richest nations were planning to shift out of the dollar and into the Euro. If this happens -and many economists say it will- the US Treasury may have to raise the rates it pays to attract investors, which means it would end up paying even more to finance it’s huge deficits. The dollar would then lose more value, and foreign investors would pull out of US markets, sending indices tumbling, and a hike in tax would follow to make up the short fall from lost sales of bonds.