The Triffin Dilemma describes how increased global trade necessitates more reserve currency, which the issuer can only provide by running deficits, eventually leading to a depletion of its gold reserves and a loss of confidence in currency convertibility.
"As international trade flows increase and economies grow, so too does the demand for the reserve currency. This demand is typically met as a result of the issuer of the reserve currency running balance of payments deficits. But if net outflows continue indefinitely, the stock of the reserve currency held abroad will eventually exceed the issuing country's gold holdings. This erodes confidence in the convertability of the reserve currency into gold, which becomes increasingly tenuous."