Fear mongering is a ploy used by anyone trying to sell you something, from an idea ("we need to go to war", "we need to stop this mosque"), to a politician, to a product for sale.
The basic trick is to link what you are selling as a curative of something dangerous, or more commonly, to highlight the dangers of not doing something. Facts are rarely used. Exaggerated statistics, links to dangerous individuals, and ambiguous language are far more useful in fear tactics.
 Famous examples
- When the US wanted to invade Iraq, General Colin Powell famously held up a small jar of powder and uttered the words "Weapon of mass destruction". There was no comment about what was in this jar of white powder, nor a statement that it came from Iraq. But it might have come from Iraq, and it might have been dangerous.
- When a Muslim group wanted to build a community center in New York, one of the many tactics used to try to prevent it, was fear mongering. "Do you really want Islamic Terrorists who have already harmed us, here?"