The currency unions which made it have all survived because they relied on a single monetary authority for managing the currency.
Counter-intuitively, single currencies are often associated with complex political entities which occupy vast swathes of land and incorporate previously distinct -and often politically, socially, and economically disparate - units. The USA is a monetary union, as was the late USSR.
All single currencies encountered opposition on both ideological and pragmatic grounds when they were first introduced.
The American constitution, for instance, did not provide for a central bank. Many of the Founding Fathers (e.g., Madison and Jefferson) refused to countenance one. It took the nascent USA two decades to come up with a semblance of a central monetary institution in 1791. It was modeled after the successful Bank of England. When Madison became President, he purposefully let its concession expire in 1811. In the forthcoming half century, it revived (for instance, in 1816) and expired a few times.
The United States became a monetary union only following its traumatic Civil War. Similarly, Europe's monetary union is a belated outcome of two European civil wars (the two World Wars). America instituted bank regulation and supervision only in 1863 and, for the first time, banks were classified as either national or state-level.
This classification was necessary because by the end of the Civil War, notes - legal and illegal tender - were being issued by no less than 1562 private banks - up from only 25 in 1800. A similar process occurred in the principalities which were later to constitute Germany. In the decade between 1847 and 1857, twenty five private banks were established there for the express purpose of printing banknotes to circulate as legal tender. Seventy (!) different types of currency (mostly foreign) were being used in the Rhineland alone in 1816.
The Federal Reserve System was founded only following a tidal wave of banking crises in 1908. Not until 1960 did it gain a full monopoly of nation-wide money printing. The monetary union in the USA - the US dollar as a single legal tender printed exclusively by a central monetary authority - is, therefore, a fairly recent thing, not much older than the euro.
It is common to confuse the logistics of a monetary union with its underpinnings. European bigwigs gloated over the smooth introduction of the physical notes and coins of their new currency. But having a single currency with free and guaranteed convertibility is only the manifestation of a monetary union - not one of its economic pillars.
History teaches us that for a monetary union to succeed, the exchange rate of the single currency must be realistic (for instance, reflect the purchasing power parity) and, thus, not susceptible to speculative attacks. Additionally, the members of the union must adhere to one monetary policy.
Surprisingly, history demonstrates that a monetary union is not necessarily predicated on the existence of a single currency. A monetary union could incorporate "several currencies, fully and permanently convertible into one another at irrevocably fixed exchange rates". This would be like having a single currency with various denominations, each printed by another member of the Union.
What really matters are the economic inter-relationships and power plays among union members and between the union and other currency zones and currencies (as expressed through the exchange rate).
Usually the single currency of the Union is convertible at given (though floating) exchange rates subject to a uniform exchange rate policy. This applies to all the territory of the single currency. It is intended to prevent arbitrage (buying the single currency in one place and selling it in another). Rampant arbitrage - ask anyone in Asia - often leads to the need to impose exchange controls, thus eliminating convertibility and inducing panic.
Monetary unions in the past failed because they allowed variable exchange rates, (often depending on where - in which part of the monetary union - the conversion took place).
A uniform exchange rate policy is only one of the concessions members of a monetary union must make. Joining always means giving up independent monetary policy and, with it, a sizeable slice of national sovereignty. Members relegate the regulation of their money supply, inflation, interest rates, and foreign exchange rates to a central monetary authority (e.g., the European Central Bank in the eurozone).
The need for central monetary management arises because, in economic theory, a currency is never just a currency. It is thought of as a transmission mechanism of economic signals (information) and expectations (often through monetary policy and its outcomes).
It is often argued that a single fiscal policy is not only unnecessary, but potentially harmful. A monetary union means the surrender of sovereign monetary policy instruments. It may be advisable to let the members of the union apply fiscal policy instruments autonomously in order to counter the business cycle, or cope with asymmetric shocks, goes the argument. As long as there is no implicit or explicit guarantee of the whole union for the indebtedness of its members - profligate individual states are likely to be punished by the market, discriminately.
But, in a monetary union with mutual guarantees among the members (even if it is only implicit as is the case in the eurozone), fiscal profligacy, even of one or two large players, may force the central monetary authority to raise interest rates in order to pre-empt inflationary pressures.
Interest rates have to be raised because the effects of one member's fiscal decisions are communicated to other members through the common currency. The currency is the medium of exchange of information regarding the present and future health of the economies involved. Hence the notorious "EU Stability Pact", recently so flagrantly abandoned in the face of German budget deficits.
Monetary unions which did not follow the path of fiscal rectitude are no longer with us.
In an article I published in 1997 ("The History of Previous European Currency Unions"), I identified five paramount lessons from the short and brutish life of previous - now invariably defunct - monetary unions:
(A) To prevail, a monetary union must be founded by one or two economically dominant countries ("economic locomotives"). Such driving forces must be geopolitically important, maintain political solidarity with other members, be willing to exercise their clout, and be economically involved in (or even dependent on) the economies of the other members.
(B) Central institutions must be set up to monitor and enforce monetary, fiscal, and other economic policies, to coordinate activities of the member states, to implement political and technical decisions, to control the money aggregates and seigniorage (i.e., rents accruing due to money printing), to determine the legal tender and the rules governing the issuance of money.
(C) It is better if a monetary union is preceded by a political one (consider the examples of the USA, the USSR, the UK, and Germany).
(D) Wage and price flexibility are sine qua non. Their absence is a threat to the continued existence of any union. Unilateral transfers from rich areas to poor are a partial and short-lived remedy. Transfers also call for a clear and consistent fiscal policy regarding taxation and expenditures. Problems like unemployment and collapses in demand often plague rigid monetary unions. The works of Mundell and McKinnon (optimal currency areas) prove it decisively (and separately).
(E) Clear convergence criteria and monetary convergence targets.
The current European Monetary Union is far from heeding the lessons of its ill fated predecessors. Europe's labour and capital markets, though recently marginally liberalized, are still more rigid than 150 years ago. The euro was not preceded by an "ever closer (political or constitutional) union". It relies too heavily on fiscal redistribution without the benefit of either a coherent monetary or a consistent fiscal area-wide policy. The euro is not built to cope either with asymmetrical economic shocks (affecting only some members, but not others), or with the vicissitudes of the business cycle.
This does not bode well. This union might well become yet another footnote in the annals of economic history.
The Concert of Europe, Interrupted
The Belgian Curtain: Europe After Communism · The Wunder Library — complete classics, free to read, with narration.