Technically Nixon didn't. He just completed the process started by FDR, who effectively ended the gold standard domestically by making it illegal for citizens to hold gold thereby eliminating the ability to convert paper to metal. Nixon just completed the process by making it so the foreign holders of paper couldn't convert paper to metal via the government. Shortly thereafter the COMEX was implemented to allowed the paper price of metal to be controlled making it unpalatable for foreign governments to convert the paper to gold. We are now witnessing the breakdown of that control mechanism.