If you start with an assumption that "alien threat comes above all else" then I can imagine building a conspiracy case on top of that with governments that hate

If you start with an assumption that "alien threat comes above all else" then I can imagine building a conspiracy case on top of that with governments that hate each other but hold hands when it comes to hiding/countering that threat. Like any other argument or reasoning, it's as strong as its weakest link, or the foundation it's built on.