well the wiki article I linked to above, discusses the issue fairly well without too much bias..
Basically the US was willing and able to assume a postwar leadership role, and it needed open markets to trade with in Europe. Britain France and the rest of Europe and Japan were devastated and had no choice anyway and were willing to tolerate US hegemony in exchange for security during the cold war.
And of course when the scheme of things no longer suited US interests they dropped the gold standard.. although by having oil bought and sold in US dollars has given it an unfair advantage the last 20years. (since when the dollar loses value - the others holding US reserves and Treasury bills also lose. That can only go on for so long.

Remember Cheney's comment that 'Reagan proved deficits don't matter'
well they do..

oh and DOugr.. the point on Americans not going to countries that use Euros kind of cuts both ways.. Now those foreigners can get a deal on overpriced US real estate...