Second, make sure your topic is manageable in size. Subjects such as ‘The fall of the Roman Empire’ and ‘The origins of the Second World War’ are the stuff of books by professional historians, who take years to write them. You should aim at a more limited and sharply focused essay topic.
Third, your essay topic should (at least in the first instance) preferably be phrased in the form of a clearly defined question. For instance: ‘To what extent was the fall of the Roman Empire caused by pressure from “barbarians” on the frontiers?’ or ‘Why did the Second World War break out in 1939 rather than 1938?’ Your essay’s argument thus naturally becomes the answer to this question.