What was a significant event in the novel American war by Omar?

I haven’t read the book, but that does indeed sound like a major event. You can certainly write about that. Good luck.

Notice that the question says ”a significant event,” not “the most.” So you don’t have to worry about arguing that the event you chose is more significant than any other. All you have to do is identify one significant event and explain why you think it’s significant.

So you have to add a statement something like this: “This event is significant because . . . ” and give a reason in terms of why it’s important within the context of the book.

Maybe instead of asking us repeatedly about this book, you should read it.

Are you asking about American War by Omar El Akkad? Because getting the author’s name right is probably a good first step in writing whatever essay you need to write. Also, you can’t take two temporally distant events and refer to them as the most significant even in the novel. And like @Jeruba said, you only need to discuss a significant event (not the most significant event). If you want to talk about the Reunification Plague, go for it. But don’t frame it as the most significant event unless you have a good explanation of why you think the ending is more significant than any of the events that led up to it.

If you actually read the book and just lack confidence about how to answer the question, congratulations on slogging through 350 pages of poorly written disaster porn. If you gave up on reading the book and are trying to use the internet as a substitute for doing your work, I’m sympathetic even if I can’t really recommend that approach.