American Civil War, the Confederate States dollar was heavily counterfeited by private interests on the Union side, often without the sanction of the Union government in Washington. The Confederacy's access to modern printing technology was limited, while many Northern-made imitations were printed on high-quality banknote paper procured through extralegal means. As a result, counterfeit Southern notes were often equal or even superior in quality compared to genuine Confederate money. In 1834, counterfeit copper coins manufactured in the United States were seized from several ships with American flags in Brazil. The practice appeared to end after. Instances A form of counterfeiting is the production of documents by legitimate printers in response to fraudulent instructions. An example of this is the Portuguese Bank Note Crisis of 1925, when the British banknote printers Waterlow and Sons produced Banco de Portugal notes equivalent in value to 0.88% of the Portuguese nominal Gross Domestic Product, with identical serial numbers to existing banknotes, in response to a fraud perpetrated by Alves dos Reis. Similarly, in 1929 the issue of postage stamps celebrating the millennium of Iceland's parliament, the Althing, was compromised by the insertion of "1" on the print order, before the authorized value of stamps to be produced (see Postage stamps and postal history of Iceland). In December 1925 a high-profile counterfeit scandal came to light, when three people were arrested in the Netherlands while attempting to disseminate forged French 1000-franc bills which had been produced in Hungary. Subsequent investigations uncovered evidence that plot had received widespread support in Hungarian and German nationalist circles including the patronage of high ranking military and civilian officials. Twenty-four of the conspirators were tried in Budapest in May 1926. Most received light sentences in what is believed to have been a deliberate cover up by Hungarian Prime Minister István Bethlen. The affair facilitated the adoption of the International Convention for the Suppression of Counterfeiting Currency in April 1929 and formalized the role of the International Criminal Police Commission. During World War II, the Nazis attempted to implement a similar plan (Operation Bernhard) against the Allies. The Nazis took Jewish artists to the Sachsenhausen concentration camp and forced them to forge British pounds and American dollars. The quality of the counterfeiting was very good, and it was almost impossible to distinguish between the real and fake bills. The Nazis were unable to carry out planned aerial drops of the counterfeits