Passive Voice

To put it technically, passive voice makes the subject of a sentence the receiver of the action. A particularly nonsensical example of this might be to write: "The bridge was hit by the car." Active voice, on the other hand, means that the subject of the sentence is the doer of the action: "The car hit the bridge." Passive voice is less concise, because it often makes the sentence longer, but it is also particularly worth avoiding in historical writing, because it can obscure who is actually doing what to whom. In contrast, writing in active voice forces you to provide more information and to explain the relationships between different actors and between cause and effect. It thus is much clearer and more precise. There are certainly circumstances when passive voice is preferable (for example, when you want to make a clear transition from one sentence to another). But more often than not, it is a way of fudging the actual historical circumstances. Consider the following sentence:

"The smurfs were attacked as dangerous radicals." (passive voice)

This leaves it entirely up to the reader to guess who actually attacked the smurfs. Whereas if you wrote:

"Many employers attacked the smurfs, thinking them dangerous radicals."

It is clear not only who is doing the action, but why.