Perfidy is a willful and deliberate breach of faith. It is prosecuted as a war crime in some regions of the world, and it can also have consequences in more ordinary civilian courts, as well. Many people regard perfidy as a particularly nasty crime because it involves gaining the trust of someone before deceiving them, and it is actually banned under the Geneva Convention, among numerous other places.
The word comes from the Latin prefix per-, which means “destruction” and fides, which means “faith,” so perfidy could be considered a literal destruction of faith. The term is usually used specifically in the context of discussions of war crimes, although one could argue that acts of minor perfidy occur in a daily basis in many regions. How many times, for example, has a seemingly trustworthy nurse assured you that an injection “won't hurt at all” if you just hold still?
There are a number of different forms of perfidy. It is probably easiest to define this term by illustrating an example. Let us say that there has been a civil war in a country divided along religious grounds. Eventually, a treaty is reached between religions A and B, agreeing to allow people of religion A to leave the country under a United Nations escort. If representatives of religion B dress up as United Nations personnel for the purpose of deceiving members of religion A into thinking that they are safe so that those people could be taken to another location to be murdered or placed in labor camps, this is perfidy.
Under the Geneva Convention, things like flying a false flag, pretending to be a noncombatant, faking an injury to gain access to enemy lines, negotiating in bad faith, and using false insignia are all considered perfidy. These actions involve a willful abuse of trust, counting on people to assume things like the idea that a ship flying an American flag, for example, is American. You could think of perfidy as deliberate deception.
Such deception is differentiated from misleading behavior under many rules of warfare. If enemy troops plant false information about their movements, for example, this is considered to be misleading, but not perfidious. While it is a willful deception, it does not rely on the abuse of fundamental rules of engagement, and in fact distributing erroneous information about troop movements is extremely common among many militaries.
The issue of perfidy has been a growing concern in modern warfare, thanks to the changing faces of military activity. In the American wars in Iraq and Afghanistan, for example, a number of members of the Iraqi military committed perfidy by pretending to be civilians, sometimes placing members of the American military in an awkward position, as they didn't know whether “civilians” were really civilians.