At the outset of a discussion of monetary integration, the characteristics that are essential for a monetary union as well as those necessary for the continued and successful existence of the monetary union must be considered.
These three requirements—effectively a single currency, a single union monetary policy, union control of international reserves and the external exchange rate—are regarded here as essential for an arrangement to qualify as a monetary union. It is necessary to realize from the beginning that the political commitment to achieve the goals of a European monetary system must be present. In other words, the national sovereignty member nations turn over to union authority and the extent national policies and performance are brought into greater harmony will determine whether a monetary union stands or falls.
Raines, J. Patrick, "The Theoretical Rationale for a Common European Currency Revisited" (1984). Robins School of Business White Paper Series, 1980-2011. 51.