A thesis statement tells your readers what your paper is about and explains what you will argue or prove in the paper.
Thesis statements have these characteristics:
It introduces your reader to an argument, something that is contestable.
It is focused. This is the difference between writing a book(s) (the reasons Vietnam War was a failure) versus one aspect you will explore in depth (assassination of Ngo Dinh Diem and resulting coup d'état).
It is not required, but some formats for thesis statements are:
They say this, but I say this
This situation may appear thus, but upon closer inspection, it is actually thus.
Now it should be apparent that you have to do research before you formulate this statement. You have to know the current conversation around the controversy and identify where you will enter.