The phrase petrodollar warfare refers to a hypothesis that one of the driving forces of United States foreign policy has been the status of the United States dollar as the world's dominant reserve currency and as the currency in which oil is priced. The term was coined by William R. Clark, who has written a book with the same title. The phrase oil currency war is sometimes used with the same meaning.
The hypothesis 
Most oil sales throughout the world are denominated in United States dollars (USD). According to proponents of the petrodollar warfare hypothesis, because most countries rely on oil imports, they are forced to maintain large stockpiles of dollars in order to continue imports. This creates a consistent demand for USDs and puts upward pressure on the USD's value, regardless of economic conditions in the United States. This in turn allegedly allows the US government to gain revenues through seignorage and by issuing bonds at lower interest rates than they otherwise would be able to. As a result the U.S. government can run higher budget deficits at a more sustainable level than can most other countries. A stronger USD also means that goods imported into the United States are relatively cheap.
Another component of the hypothesis is that the price of oil is more stable in the U.S. than anywhere else since importers do not need to worry about exchange rate fluctuations. Since the U.S. imports a great deal of oil, its markets are heavily reliant on oil and its derivative products (jet fuel, diesel fuel, gasoline, etc.) for their energy needs. As the price of oil can be an important political factor, U.S. administrations are quite sensitive to the price of oil.
Political enemies of the United States therefore have some interest in seeing oil denominated in euros or other currencies. The EU could also theoretically accrue the same benefits if the euro replaced the dollar. However, the European economy could also be seriously damaged if the euro were to appreciate significantly against the dollar or other world currencies, particularly its exports which would become relatively more expensive for the rest of the world. The same dynamic can apply to the dollar and the U.S. economy as well.
Political events 
The Government of the Islamic Republic of Iran takes this theory as fact. As retaliation to this policy seen as neoimperialism, Iran has made an effort to create its own Iranian Oil Bourse which started selling oil in Gold, Euros, Dollars, and Japanese Yen. In mid-2006 Venezuela indicated support of Iran's decision to offer global oil trade in the euro currency.