I'm presuming that you mean a traditional (full) essay type question.
1] to tell a story
2] to tell a good story
3] to tell a good story well
You move from 1 to 3 step by step with practice.
This is where your plan comes in - a good plan is half the battle and well deserves half the time. It is not a quick list of anything you can think of which has to be got out of the way quickly before you start writing.
Traditionally a history essay would start with an introduction and finish with a conclusion (based only on what you have put in the body of the essay). The body of your essay contains your points or evidence and is divided into paragraphs. You can use 2/3 paragraphs if your dealing with an important point or issue.
The introduction outlines your case or what you are going to discuss. You can take a position if you want e.g. "the outbreak of WW1 was inevitable given the prevailing circumstances." Perhaps soften it to "almost inevitable".
Your essay would then go on to explain your position in subsequent paragraphs. Describe, explain and justify with examples. Include opposing or alternative view(s). Use and quote sources as required by your school/college.
Probably a good idea to build your essay in terms of importance or impact. Very much as you would see in a movie.
Then you set out your conclusion around the points you have presented.
It's practice and perseverance that will improve your essay style and that takes time.
P.S. a really good understanding of Imperialism is essential to History. It runs through the Ukrainian war and is fundamental to modern US policy.
Finally it would be useful to have a good grounding in the reason for war and it's benefits to nation states.