I don't know much about the American civil war but I know how to write an essay.
The question say to "assess the extent" so you have to make a judgement on how significant it was. A common trap students fall into is that they will try to answer the question only talking about slavery. In order to answer this question fully, you need to talk about other factors which led to the civil war (I ave no idea what they would be so I hope you do) and then say that while slavery was an inmortant factor, it isn't the sloe cause of the civil war.
As for structure, so you've got your basic intro-body-conclusion thing happening. In your intro write a brief statement about the significance of slavery and mention the other factors.
Next paragraph, talk about why slavery was imporant. So you write your topic sentence first, then you go into detail. Remember to give examples and talk about events and issues specifically rather than just waffle on. I ggives you more credibility.
In the following paragraphs, follow the same structure as the slavery section but wtih other issues.
Finally conclude your essay by restating your thesis.
Remeber to write more about slavery than the other factors because it is specified by the question but talk about other factors in adequate depth as well.
This is a general way to answer any 'assess the significance" questions and can be applied to other topics in the modern history course.
