Thesis: American Literature has been impacted by many things. But nonetheless, WWII had one of the biggest impacts.
Then start writing why it left an impact-this part isn’t your thesis. Then, you’re gonna write your body paragraphs. At the end, when writing your conclusion, restate your thesis that I wrote up at the top.
p.s if you don’t know how to write your thesis, try using words about the topic instead of reasons-since you’ll use the reasons in your body paragraphs. So instead of having your thesis be “American literature was impacted by WWII because….” Write something like I did at the top-hope that made sense haha.