US Dollar took over as the world currency thanks to Bretton Woods 1944 at the end of the World War II. With the world facing tremendous calamity, a sensible system was put in place, incorporating the learnings from the failures of Treaty of Versailies.
Begining its inception, the European states set out on a development spree that has since remained unmatched. Even the rise of China in peacetime does not match the speed and quality of development. But with development, there appeared cracks in the Bretton Woods system. Well, not exactly the Bretton Woods system - but the currency system. There appeared an fundamental incompatibility between the unique construction of US Dollar and the structure of Bretton Woods.
The Construct of US Dollar
US Dollar created in the aftermath of American struggle for independance was gold backed as was required to be able to trade in the global system. At the early stage, US Dollar was indeed made out of gold-silver mix (1:15) and each dollar was backed by 1.60gm of gold. The gold-silver ratio was reduced to 1:16 thereby devaluing the gold equivalence which now came to 1.5gm gold for each dollar. The new coins were made by gold-silver mix so exact devaluation can be debated. However, the weights of the coins continued to be reduced over the 19th century.
This bimetallism had to be gradually dropped since the silver coin weights were reduced and later substantial silver deposits were discovered leading to wild price fluctuations. Thus US Dollar came to exclusive gold standard. The formal gold standard act backed the Dollar with 1.67gms of Gold - a smaller devaluation by itself.
Note that at the time, Pound Sterling was the dominant currency and it was exclusively backed by gold. The Pound also fluctuated in its gold peg till early 19th century where the British put in place the gold standard. Their wide-spread empire and british respect for the value of the pound contract meant it quickly became global currency. US Dollar was emulating this precedent.
In this process the pegs to the gold were altered by World War I and subsequently in 1931 Britain gave up the Gold Standard, leaving American Dollar as strong contender for world currency. All this was formalized in 1944 at Bretton Woods.
The Second part of Bretton Woods Agreement
At Bretton Woods it was also agreed that exchange rates between some dominant currencies would be pegged to the US Dollar and they were backed by Gold. With such a policy in place, European countries started on most ambitious reconstruction plan. This was supposed to be an opportunity for the American companies and it was. But it also created many European companies who became competitive vis-a-vis their american counterparts and started a cross-flow of trade and commerce. In the process, more US Dollars were created and soon there were too many and not enough gold to back it.
This fact was noticed by the trading community who started bidding up the Gold prices prompting John Connally to push for abandoning the gold standard. He said later "My philosophy is that all foreigners are out to screw us and it’s our job to screw them first."
The Fiat-Dollar era
The Fiat Dollar continued its run as the global currency thanks to burgeoning US population, US growth and demand from US markets. International trade soon became thoroughly Dollarized. The dollar-peg concept went from Europe to Japan. The latest in that phase came the dollar-peg by East Asian Tigers and mainly China. These countries vountarily gave up their freedom to conduct their monetary policy - depending on prudence of the US FED. This system then created today's unique problems.
Now with US being the defacto currency in the world, US lost its ability to devalue. Faced with this situation, US followed what John Connally famously said "The dollar is our currency and it is your problem". It printed and printed and printed. And so did everyone else - by default. In effect we do not see US Dollar being devalued - the point gold bugs keep making. Without the devaluation US is not getting the turbo-boost to kick start the growth leading many to call the other countries' monetary policy as "predatory". This problem is a corollary of the famous "Impossible Trinity" or "Mundell-Fleming Trilemma".
What should be the decent international currency system?
It is now clear that monetary policy independance can be given up volantrily and also taken away by coordinated action. The solution many propose is to go back to Gold standard - which may be a good intermediate arrangement - but not a good long term arrangement. A better idea is to go for a two-level currency system. SDR may be a good starting point - but SDR's may not give us the true global currency we need.
The global currency and relatively-fixed (stable) peg to global currency could be a good system. It will leave the monetary policy freedom with national central banks and yet keep the system stable most of the time.