The best way to structure a History essay is usually thematically. For example, if a question is framed such as 'How successful was the Civil Rights Movement in America?' you should divide your essay into three themes. For example, you could discuss economic, political and social impacts. For top-end answers, you should really try to dissect the question as much as possible. Think about the term 'successful' and how you can form an interesting article out of this: is the success of the movement greater in the long-term or the short term? Was it more successful in different areas? Was the concept of the movement inspiring, but was not executed effectively enough by civil rights leaders? 'Success' can be of varied measures, and is not usually a straight answer.
Plan a solid introduction too. The best introductions begin with a couple of lines of context explaining the source of debate and in what climate it occurred in. Then you should define the key words in the question, to bring in the interesting points of debate into your essay early on. Using our example, these would be 'success', which as we have seen has many different measures, and you might want to define 'Civil Rights' too, as there are disputes over what the Civil Rights Movement actually characterises. After your definitions, you should then introduce your thematic approach and make it clear to the examiner how your essay will be structured. Start with your weakest theme and end with your strongest. At the end of your introduction, briefly summarise your main argument so it is clear from the beginning what you are arguing. One of the most crucial parts of an essay is clarity and consistency.
