I imagine militaries would have a fairly easy time seizing power in nations with few firearms, due to few civilian in most nations having suitable weapons to resist, and seizing power could help the military by allowing them to pass laws that benefit themselves at the expense of others, such as by having extremely heavy taxes to use to become wealthy. Though other nations' militaries could try to prevent a military dictatorship from forming, this seems unlikely because all of the nations' militaries could team up to set up a world dictatorship.
So, why don't militaries all form dictatorships? Are they simply too nice to? Is there some reason why they can't?