Many different factors contributed to the Nationalists' triumph over the Republicans in the Spanish Civil War, and the question is asking you to explore the impact and significance of these factors on the ultimate outcome. The main factors I'd explore are the political, the economic, the military and the role of foreign intervention, backing up each point with statistics and historiography where appropriate. It is equally important to compare these factors, so when drafting your essay, you want to decide which one or two points were the key reasons for the Nationalist victory, and which ones were less significant. In your introduction, it is important to clearly indicate the factors you will explore and the ultimate verdict you have come to in terms of the relative significance of the factors. Then, in your main body paragraph on political reasons, you could discuss how the Nationalists were a lot more unified under Franco's leadership, which can be explained in terms of a lack of other potential leaders (particularly following the death of Mola in 1937), the role of the Church in creating unity, and the use of propaganda in Nationalist territory. Conversely, Republicans were more divided, as could be seen from the different, often competing sub-groups with clashing objectives (such as the liberals, the socialists and the anarchists), which often resulted in infighting such as during the 1937 May Days in Barcelona, which resulted in the death of around 700 people. You could then argue that these political divisions were a key reason for the Nationalist victory because there was an insurmountable gap in effectiveness of organization and that infighting particularly aggravated the situation, or you could say that the political divisions, while putting the Republicans at a clear disadvantage, were not as important as other factors, such as the role of foreign intervention. You would then proceed to discuss the other reasons for the Nationalist victory, clearly indicating at the end of each paragraph whether the reason being discussed is more or less significant in explaining the outcome. The conclusion serves to summarize the main points discussed and to reiterate your thesis, and you could finish by broadening the essay’s horizons by alluding to how the Spanish Civil War seemed to foreshadow other impeding conflicts and divisions in Europe.