Forgery as covert operation

Forgery is used by some governments and non-state actors as a tool of covert operation, disinformation and black propaganda. Letters, currency, speeches, documents, and literature are all falsified as a means to subvert a government's political, military or economic assets. Forgeries are designed to attribute a false intention and aspirations on the intended target. They force the targeted government to spend a large amount of resources to refute the forgery. Forgeries are an effective tool because of their ability to hold influence even after being proven false.
Techniques.
Counterfeit money.
Counterfeiting is the creation of currency without the official sanction of the issuing state or government. It is designed to look like the real currency and convince others of its value. Counterfeiting has been used as a tactic to increase individual wealth since the creation of currency itself. Counterfeit money has also been used as a covert operation by rival nations in an attempt to create inflation and subvert. Counterfeiting is a useful tool of subversion because it results in a reduction in the value of real money; an increase in prices (inflation) due to more money getting circulated in the economy - an unauthorized artificial increase in the money supply; a decrease in the acceptability of paper money; and losses, when traders are not reimbursed for counterfeit money detected by banks, even if it is confiscated.
Operation Bernhard.
Operation Bernhard was a counterfeit campaign conducted by the Nazi regime to flood the United Kingdom with £5 and £50 banknotes. The scale of the counterfeit campaign was the largest ever conducted since the advent of paper money. The original plan was to introduce the counterfeit money into Great Britain to create inflation, but this plan was abandoned due to diplomatic issues. Rather, the counterfeit notes were used to finance war expenses. The counterfeited notes were created at the Sachsenhausen concentration camp by prisoners who were experts in forgery.
Philatelic forgeries.
Philatelic forgery is the creation of false postage stamps that are designed to look genuine. Because of their wide publication, stamps are a powerful tool of propaganda. Forged stamps were used by both the Allies and Axis Powers during World War II. Large philatelic campaigns were also conducted throughout the Cold War.
Literary forgery.
Literary forgery is a body of written work attributed to a certain eminent, historical or popular author. This forgery is often a fake manuscript or diary created to attribute certain ideological beliefs or actions to the figure. Literary forgery is often difficult to refute because the purported author is usually deceased.
United States Secret Service.
The United States Secret Service is the agency within the United States' government tasked with the protection of U.S. currency. The Secret Service has jurisdiction over violations involving the counterfeiting of United States obligations and securities. Some of the counterfeited United States obligations and securities commonly investigated by the Secret Service include U.S. currency (to include coins), U.S. Treasury checks, Department of Agriculture food coupons and U.S. postage stamps.
The United States Secret Service was created by Abraham Lincoln during the Civil War in 1865 to combat the high counterfeit rate of currency. At the time, one-third to one-half of all U.S currency in circulation was counterfeit. Today, the Secret Service continues its core mission by investigating violations of U.S. laws relating to currency, financial crimes, financial payment systems, computer crimes and electronic crimes. It utilizes investigative expertise, science and technology, and partnerships to detect, prevent and investigate attacks on the U.S. financial infrastructure.