I've touched on this in a bit on a previous answer I did for Imperial Japan's trading policy in the lead up to World War II. While Japan could use Japanese yen to finance trade within its Empire and occupied territories, when it came to purchasing goods from overseas, much of their dealing was in U.S. dollars. One of Japan's major source for dollars was, of course, gold sales to the U.S. Treasury, along with US$ gained from Japan's own exports to the U.S. and other financial instruments. Relatively early on in the war, many of the European countries introduced strict currency controls, meaning that British pounds sterling, French francs, and Dutch guilders--many of which had been used on international markets prior to the war--were suddenly much less useful, making the U.S. dollar the de facto trade currency of the world at that point. It also helped that, as far as war materiel was concerned, most of the European powers were no longer going to be selling such products on the global market, instead focusing on using those goods to fuel their own war efforts. This helped reinforce the prominence of the U.S. dollar as the U.S. was pretty much the only economy selling many goods as the war in Europe kicked off.

This is not necessary my main topic area, but I hope this and my linked answer helped. Please feel free to ask any follow ups, and I'll do my best to answer them.