We already have enough firepower with standard nuclear warheads to destroy civilization a few times over. I doubt a bigger bomb would fundamentally change the equation at all.
A fusion bomb necessarily needs a transient, extreme ignition, which I believe can only be achieved with a fission bomb (even with the truly massive fission energy building an H-bomb took a while!).
>A fusion bomb necessarily needs a transient, extreme ignition, which I believe can only be achieved with a fission bomb
Inertial confinement doesn't really need much energy. It needs huge power, huge energy density (notice that power != energy). Fission primary is the best way to achieve that of course, yet Sandia Z-machine or LNL NIF lasers achieve power enough for ignition too while not spending any noticeable amount of energy (NIF uses old lasers so it consumes more energy than it would if it was built with today's lasers). Both devices - Z-machine or a NIF-like with the old lasers replaced by modern solid state ones - are already of the size on the scale of 10-20 shipping containers and are capable of burning something like a grain size pellets which creates neutron flux deadly in the area of the size like that LNL NIF building. To me it seems like there are really great chances that either of them would be miniaturized enough to produce a space engine as well as a weapon (to which our civilization doesn't look ready).