Mao said that power flows from the barrel of a gun. He also said (and this is less remembered) that therefore, the Party must control the gun, and the gun must not control the Party. In other words, the brute facts of violence are important, but so are the social arrangements that control them.
This has been true throughout most of history. Whoever has control of violence will tend to gain political power. In several times and places, the military did not actually rule, but submitted to a legitimate authority—the United States is a decent example of this, or most the the European powers in the last few decades. But more frequently, those with the means of violence make the rules. Recent events in Egypt and elsewhere bear this out, as if we needed more examples.
That said, it makes a huge difference what the state of military technology is. For that will determine if weapons are available to the mass of people, or if they are restricted to only an elite few. Samuel Finer argued that his monumental History of Government (now out of print, and sadly hard to get—inexcusable on the part of Oxford Press in this time of print-on-demand!) that when weapons were widely available, politics tended to feature mass participation and broad egalitarianism, if not outright democracy as in the case of ancient Greek city-states and their hoplites. (Or, one might add, early America.)
On the other hand, when specialized weapons gave advantages to those wealthy enough to afford them, power tended to be concentrated in the hands of a few. For example, the rise of powerful kings in Europe had much to do with the advent of cannon—fantastically expensive to make, requiring a large specialized infrastructure of foundries. Furthermore, with cannon French kings were able to reduce the fortresses of their rebellious nobles, consolidating their own power.
In an earlier age, the armored knight was the undisputed master of the battleground, able to crush unarmored opponents with ease. Thus, power tended to be held by the armored warlords of the feudal era, whose rule depended on their use of naked force. Then the free Swiss militias developed their famous style of pike warfare, which completely nullified the advantages of the knight.
So weapons technology played a large role in politics. When considering a given era, we must ask: how common are weapons? Are they easy to use, or do they require specialized training? Do the wealthy gain any particular advantage from their wealth, or can mass armies defeat them?
This line of argument is one of the bases of the American gun-rights movement (examples can be found here, here, or here, but there are many others). It was also argued by Max Weber that the rise of the Israelite kings (over a previously egalitarian society) was the result of advanced armor, which gave a significant battlefield advantage to those wealthy enough to buy such armor.
This reasoning can also help explain the rise of child soldiers. P.W. Singer argues that child soldiers are now more feasible because small arms are becoming more advanced and lighter. Children can now use weapons effectively on the battlefield in spite of their small size and physical weakness, which has not been true for hundreds of years if ever. As a result, child soldiers are becoming a frequent sight in war-torn areas, since it is relatively easy for a brutal would-be warlord to coerce children into fighting for him (or her, I suppose).
Similar issues are beginning to arise because of drone technology. Robots have often been used for fun by hobbyists; but it is only a matter of time before these can be weaponized, and made available off the shelf. Governments will be unable to stop the spread of drone weapons into the general populace, and the social effects of this shift are likely to be extreme.
So as a writer, how do you use this?
First of all, when you are world-building, be careful to compare the state of weapons technology with the social system. Kings and castles are unlikely when no one wears armor or carries swords, or if everyone does. Magic can also be a weapon, in this sense, so if powerful magic is rare, it should generally translate into considerable power (unless there are social reasons otherwise).
You can write an interesting story about social upheavals caused by changing technology. For example, I’m presently messing around with a story where magic previously relied on using decades-long mental training to draw sigils of power in your mind; but then someone figured out how to get the same effects with sigils carved into physical media, such as discs of wood, and then everything collapsed into chaos as weapons technology exploded into the populace.
A great example of this concept is in the early installments of the excellent webcomic Schlock Mercenary. A new means of transport allowing for functional teleportation is rapidly weaponized, and bombs are teleported into government offices across the galaxy. Chaos and war break out on hundreds of planets, and things only die down when scientists figure out how to block teleportation into protected areas.
I hope this piece proves useful. At any rate, it should be food for thought.