(This post is part of Politics for Worldbuilders, an occasional series.)

Being a ruler can be hazardous to your health. The safety of your country depends on you having a strong enough army to repel invaders, and the safety of your regime depends on that same army being able to deter rebellion. But an army strong enough to do that is also strong enough to overthrow you by itself; history is full of ambitious generals who did just that. So what is a ruler to do?

There are many strategies that can be employed, such as keeping your soldiers fat and happy with tax revenue, or using ideological indoctrination to secure their loyalty. Here, I want to focus on army composition, and how it can be used to secure the regime.

By “army composition,” I don’t mean how much infantry you have versus cavalry, or battle mages versus dragons, or whatever—though that is clearly important. And if you do want to think about that kind of thing, questions of unit type can easily fit into the model we are about to discuss. But as Samuel Finer discusses, a ruler fundamentally must build his military from among three kinds of armies: popular militias, a professional national army, and foreign mercenaries.

The popular militia is the cheapest and easiest option, if your objective is to defend against invasion (or, sometimes, to do a spot of invading yourself). Responsible for their own training and equipment, the populace does not represent a drain on the treasury as other types of soldiers do, and they can be raised quickly when needed. However, they tend to be relatively poorly trained and armed, and are therefore less effective in battle than a standing army. More importantly, to the ruler, is that the popular militia is loyal to their families foremost, their nation second, and the regime a distant third—if at all! Especially if you plan on being a squeeze-the-peasants sort of ruler, allowing the people to organize into armed units would be the last thing on your mind.

A standing army remedies many of the defects of the militia. Soldiers are better equipped and better trained, dependent on the regime for their pay, and also more easily indoctrinated politically (if that kind of thing is a feature of your regime). However, professionalized armies take a long time to train up, and are fantastically expensive; Finer estimates that the vast majority of state spending throughout history was on maintaining armies. Moreover, while soldiers in a standing army may be more loyal to the regime than your average peasant is, they will still care more about the nation as a whole—and might decide that the ruler needs to go for the public good. Alternatively, one of your commanders may decide that he wants your job, and convince his men to back him. A standing army thus represents a permanent threat to the regime, more urgently than the populace as a whole does.

Finally, we have mercenaries—they come pre-trained, only care about getting their pay, and have no sentimental attachments to the populace. Indeed, the populace may view them with resentment or hatred, deepening the mercenaries’ dependence on the ruler who signs their paychecks. On the other hand, mercenaries are notorious for their lack of fight-to-the-death commitment, are prone to switching sides, and even are known to overthrow the regime and take over—as Machiavelli notes in great detail when discussing the Italian Condottieri.

One classic way that tyrants would mitigate these risks is for the ruler to have a small number of mercenaries as his personal guards. They would not be strong enough to credibly challenge his rule, and would be at risk of massacre by the locals if the ruler ever died; but they can still keep him safe from the standing army, and indeed their own welfare depended on it.

Likewise, most regimes would maintain a relatively small standing army as the core of the military, along with a much larger popular militia to be called up during wartime. The standing army would serve as shock troops in battle, improving the effectiveness of the military as a whole, and would also defend the capital city from any restiveness in the outer provinces—all for a relatively low cost, compared to an entirely professionalized force.

Of course, a wealthy enough regime that was not loathed by its people could have the luxury of a powerful full-time army; the United States is one example today. But even the U.S. still maintains a distinction between the Army and the National Guard, which could be seen as a rough parallel to the “popular militias” discussed above. A better example would be the Iraqi army under Saddam Hussein; the bulk of the military was poorly equipped and paid, while the smaller Republican Guard—recruited exclusively from Hussein’s own clan—was a relatively elite force whose performance, and loyalty, were more assured. Meanwhile, several small African or Pacific Islander states relied in recent years on mercenary groups such as Executive Outcomes or Sandline, because they were viewed as more reliable than the regime’s own military. With good reason; these regimes tended to have a long history of military coups.

So the militia/standing army/mercenaries model is broadly applicable. In your worldbuilding, consider the strategic problems that a regime faces from its own military. They are sure to generate some gripping stories, if you want to write them. Key points to consider: the cost of an army, its loyalty, the loyalty of its commanders, the need for more loyal units to deter mutiny by the more marginal ones… and trading off all of the above against military effectiveness. Remember too that some rulers can more easily survive defeat in war than they can a military coup—and will therefore treat their own military as their greatest threat.

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(And don’t forget, I’m accepting submissions to a fantasy anthology, Ye Olde Magick Shoppe. There are only two days left! Check out the announcement and start writing!

Plus, the associated Kickstarter project is now live! We’ve got a fancy video and everything…)