If you point a gun at someone they don't necessarily do as you wish. In military situations the personnel are trained (some might say brainwashed) to sacrifice themselves; which should be relatively easy when the alternative is that all your family die from radiation poisoning (or obliteration if they're lucky).

Arguably in some regimes chain-of-command is a greater incentive than a gun as retaliation against families (for "treason"/cowardice) seems reasonably common.