[arethuza]

Cobalt salted hydrogen bombs are one fairly plausible route - how many and how large they'd need to be is another question.

https://en.wikipedia.org/wiki/Cobalt_bomb

[Of course, they don't exist at the moment - but they wouldn't be that difficult to make as one advantage of a Doomsday bomb is that you don't need to deliver it so it can be made as large as you want through staging].

[tome]

Interesting. So the idea is that a small group of people could unilaterally build and detonate one of these without anyone else being able to stop them?

[arethuza]

In David Brin's novel Earth World War 3 is basically the world vs Switzerland - with the Swiss threatening to detonate large cobalt bombs buried under the Alps.