Today's polisci classes were interesting. Among other things, we discussed the Offensive-Defensive Arms Balance component of defensive neorealist theory. Jargon aside, the basic idea is that a given weapon system is has an inherent value for defense and a value for offense. Minefields, for example, are almost completely defensive. Tanks, though capable of both offense and defense, generally are intended for offense; in WWI, the tank was the weapon that cracked Germany's trench-network.
Two theories are proposed. First, that the state of technology will favor either defense or offense. (At the beginning of WWI, trench warfare and the machine-gun made offensives bloody and generally futile. By WWII, warfighting technology had shifted far enough to the offense to permit the success of the German Blitzkrieg, and later the D-Day landing, in the face of every conventional defense available. Today the balance is even more towards the offense, I believe.) If the balance is toward defense, so goes the theory, states will find it too costly to go to war and will generally tend toward peace. If the balance is toward offense, no state will dare risk falling victim to a surprise attack, and will have an incentive to attack first, thus tending toward war. (The theory failed disastrously in WWI and before WWII because Europe's leadership didn't take the new technology into account, except for Nazi Germany.)
The second theory, which I think is more interesting practically, is that states send signals with their armaments. If a state has a military that is primarily defensive, surrounding states will understand that the first state does not mean to attack them. They will thus refrain from needless arms buildups which may increase the chance of war. Conversely, states with an offensive military will tip off their neighbors to prepare for war.
In essence: All else being equal, the presence of purely defensive weapons will influence a region towards peace.
Which brings me to the citizen's militia.
I believe in the benefits of private ownership of weapons. It acts as a check on tyranny, inspires self-reliance, and makes violent crime much more dangerous and less attractive. Many have argued that in an age of standing armies, private gun ownership is superfluous. (Never mind that standing armies tend to produce tyrannies...) But defensive neorealist theory would indicate that it is to the benefit of the state to have an armed populace, since citizen's militias are relatively cheap and easy to set up and are entirely defensive in character. They create a powerful deterrant to invasion, and thus allow the conventional military to be kept small and non-threatening. (Consider the case of Switzerland, where every household has a SIG Sauer assault-rifle.)
It is in the interest of world peace that I argue that every state should arm all of their citizens. It would also restrain corrupt governments quite effectively; unfortunately, we will see very few militias for that very reason.
If you want to get worried, think about the criminalization in the U.S. of private militia groups.