If the first syllable of a word with a pronounced h is stressed the proper article is a. Otherwise it is an.

Examples:

A) He wrote a history of World War Two.

2) They put on an historical re-enactment of the First Battle of Bull Run (Sorry, I couldn't spell Manassas)

Note: Application of this rule may vary from place to place. Please consult all pertinent state laws before attempting to follow this rule. The management takes no responsibility for mis(ab)use of this rule.