We were spending a lot of money on Vietnam and the Great Society programs as a result of LBJ's guns-and-butter policies. According to the Bretton Woods agreement of 1944 we were supposed to keep the dollar at the rate of $35 per ounce of gold. To do that while we were overspending we created the London Gold Pool, which was a clever way of keeping the worldwide price of gold down. But the French got wise, and DeGaulle decided to ask for gold in redemption for dollars. (Don't dump on DeGaulle, because it was the prudent thing to do.) The gold drain got bad enough that in 1971 Nixon closed the gold window, and we went to floating exchange rates. Treasury Secretary John Connally said to the Europeans, "They may be our deficits, but they're your problem".
A good summary.