The first phase began with the Bretton Woods agreement in 1944 and the expansion of the dollar as the post-WWII reserve currency that regime put into effect. The BW agreement made the dollar the de facto world reserve currency for payments as long as those payments could ultimately be extinguished through final settlement in gold through the central banks.
It worked fairly well. However, it allowed the US to effectively get a free ride to produce too many dollars to fund its defense spending and expand its own economy. It was a great deal so long as that dollar production did not end in too much final settlement in gold.
However, inevitably, by the end of the 60s foreign central banks, especially in Europe, were visiting the gold window at the US Treasury way too often seeking final settlement in gold