Convinced of an existential threat from competitors, America is weaponising the US dollar to preserve its global economic and geopolitical position.

While the US accounts for about 20 per cent of the world's economic output, more than half of all global currency reserves and trade is in US dollars. This is the result of the 1944 Bretton Woods agreement, the effect of which was enhanced when the link between the greenback and gold ended in the 1971 Nixon shock, allowing America to control the supply of the currency.

Currency warfare: The US dollar's position as world currency gives America disproportionate power. Phil Carrick

The dollar's pivotal role - an "exorbitant privilege," in the term coined by then French Finance Minister Valéry Giscard d'Estaing in 1965 - allows the US easily to finance its trade and budget deficits. The nation is protected against balance-of-payments crises, because it imports and services borrowing in its own currency. American monetary policies, such as quantitative easing, can influence the value of the dollar to gain a competitive advantage.

Weaponising payment flows

But the real power of the US dollar is its relationship with sanctions programs. Legislation such as the International Emergency Economic Powers Act, the Trading With the Enemy Act and the Patriot Act allow Washington to weaponise payment flows. The proposed Defending Elections From Threats by Establishing Redlines Act and the Defending American Security From Kremlin Aggression Act would extend that armory.

When combined with access it gained to data from Swift, the Society for Worldwide Interbank Financial Telecommunication's global messaging system, the US exerts unprecedented control over global economic activity.

Sanctions target persons, entities, organisations, a regime or an entire country. Secondary curbs restrict foreign corporations, financial institutions and individuals from doing business with sanctioned entities. Any US dollar payment flowing through a US bank or the American payments system provides the necessary nexus for the US to prosecute the offender or act against its American assets.

Our currency, but your problem. US President Richard Nixon's Treasury secretary John Connally Jr.

This gives the nation extraterritorial reach over non-Americans trading with or financing a sanctioned party. The mere threat of prosecution can destabilise finances, trade and currency markets, effectively disrupting the activities of non-Americans.

The risk is real. BNP Paribas paid $US9 billion ($12.5 billion) in fines and was suspended from dollar clearing for one year for violating sanctions against Iran, Cuba and Sudan. Other banks including HSBC Holdings, Standard Chartered, Commerzbank and Clearstream Banking have paid large fines for similar breaches.

Secondary sanctions made it difficult for United Co. Rusal to refinance dollar borrowings when global businesses, banks and exchanges were forced to stop dealing with the Russian company.

Its bonds and shares plunged, even though the company sells only 14 per cent of its products in the US, does not use American banks, and is listed in Moscow and Hong Kong.

ZTE Corp., a Chinese electronics company, was hit hard by the inability to buy essential components from suppliers because of sanctions for trading with North Korea and Iran.

In these cases, the entity was not in violation of laws where it was domiciled or operated, and the proscribed acts took place outside the US.

No way out

China, Russia and increasingly Europe want an alternative reserve currency system. The problem is that immediate replacement of the dollar is difficult.

First, the euro, the yen, the yuan and the ruble are not realistic options. The euro's long-term future and stability isn't assured, while Japan's economy remains trapped in two decades of torpor. The Chinese and Russian political and economic systems lack transparency, and the yuan isn't fully convertible.

Second, the required change in infrastructure is daunting. Foreign-exchange markets where the US dollar is the currency of reference would have to be fundamentally restructured. Deep and liquid money markets to support a reserve currency can't be conjured up overnight.

Third, most candidates are reluctant to take on the role of a global reserve currency because of tensions between national and global economy policy. The economist Robert Triffin pointed out that the country whose medium of exchange is the global reserve currency must meet external demand for foreign exchange. This necessitates running large trade deficits, requiring fundamental changes in the mercantilist policies of Germany, Japan and China.

This means that the US can continue to use its currency to help further its trade, financial and geopolitical aims, largely outside the strictures of international laws and institutions and without the need for messy, unpredictable military campaigns. As John Connally Jr., Richard Nixon's Treasury secretary, put it in 1971: The dollar is "our currency, but your problem."

Satyajit Das is a former banker, and the author of the books "A Banquet of Consequences", "Extreme Money" and "Traders, Guns & Money." He lives in Sydney.

Bloomberg