Students should consider that the question implies that there is more than one cause here, and so should demonstrate some form of knowledge that perhaps the causes interconnect, and that the war was not because of one sole reason. Also, as the question asks you to "evaluate", students should be thinking which one was the most important, or the main turning point that made the war happen. You should also consider who was more responsible, as the war can be seen to be caused from many sides (e.g. the USA, the USSR, or North/South Korea). Your main paragraphs should consider one cause each, e.g. 'US aggression was the most responsible for causing the Korean War' (note here the use of "most" to signify that you know that there are other forces at play). You should then give at least 3 pieces of evidence as to why you argue this, and then finally sum up each paragraph by explaining how this evidence supports your argument, perhaps acknowledging the wider context and how this fits your argument too.