The roles of a reserve currency are to finance international trade and to function as a store of value for Governments. Until the second world war it used to be the British pound, but with the demise of the British Empire, the pound lost its international relevance and was overtaken by the dollar. This was formalized in the 1944 Bretton Woods system. All other currencies were fiat currencies, but pegged to the dollar, which in turn was pegged to Gold at 40 dollars an ounce and redeemable for international trading partners.
With the dollar as the reserve currency, the US had to export dollars. In the early years after the war especially for Europe, the famous Eurodollars. This sounds great: print money and buy whatever you like. But with the Gold window it was also risky: overprinting could mean excess dollars would be exchanged back to Gold, depleting US Gold reserves.
This was also a weakness that those annoyed with American Hegemony could exploit. In 1967 the leftist press mogul Jean-Jacques Servan-Schreiber penned a famous screed called ‘le défi Américain’ (the American challenge’), arguing Europe was being colonized economically by superior American competition.
France, at the time, was run by de Gaulle, who never was impressed with Anglo-American supremacy. He made a point of exchanging every dollar he could lay his hands on as a means to undermine it.