Spies have been around as long as the state has existed. War strategists have praised the utility of spies going back to Sun Tzu's "The Art of War" for their ability to augment or replace other military elements, saving lives and sometimes preventing battle altogether. Sun Tzu described five types of spies: the local spy, the internal spy, the converted spy, the permanent spy and the expendable spy. Expendable spies are spies whose fate does not matter, so they can be used to do things that could get them killed or captured, and because of this, Sun Tzu touts the expendable spy who does not know he is expendable as the best kind of spy. Sun Tzu goes on to say that local spies are enemy peasants, internal spies are enemy courtiers. Converted spies are what we would call "double agents," and permanent spies are more like what the CIA calls "case officers," communicating intelligence back to headquarters.
History is full of spies, and many of the best will, by their nature, never be known. There are some great spies whose stories have since come out, but who were not known, or at least not known to be spies until long after they were laid to rest. Here are 25 you may not have heard of.