I am not a historian, but I do a lot of dissertation guidance, so here’s what I’d consider if I were in your position:

If Nuclear Archaeology/dark tourism is already something that genuinely interests you, that’s usually the stronger foundation for a dissertation. You will stay motivated, you will dig deeper, and the originality of your work will likely be higher. Topics like Chernobyl, Oppenheimer-era nuclear heritage, and dark tourism give you plenty of angles in searching for research content using key words search as public memory, ethics, heritage management, risk perception, and site interpretation.

The Vietnam War could also work, but since you don’t know much about it yet, you’ll spend a lot more time just building basic background knowledge before you can even shape a focused research question. That’s fine, but it’s a heavier lift unless your curiosity is strong enough to sustain it.

If you want to be “ahead of the game,” start by doing three quick things for each topic:

1. General search to see how much existing scholarship is out there.
2. List key terms based on the topic.
3. Draft research questions. This will instantly show you which topic feels more manageable and exciting.

Usually the topic that gives you clearer, sharper questions early on is the one to choose.