Justice Joseph Story: Second Amendment
"One of the ordinary modes, by which tyrants accomplish their purposes without resistance, is, by disarming the people, and making it an offence to keep arms, and by substituting a regular army in the stead of a resort to the militia. The friends of a free government cannot be too watchful, to overcome the dangerous tendency of the public mind to sacrifice, for the sake of mere private convenience, this powerful check upon the designs of ambitious men."
"The importance of this article will scarcely be doubted by any persons, who have duly reflected upon the subject. The militia is the natural defence of a free country against sudden foreign invasions, domestic insurrections, and domestic usurpations of power by rulers. It is against sound policy for a free people to keep up large military establishments and standing armies in time of peace, both from the enormous expenses, with which they are attended, and the facile means, which they afford to ambitious and unprincipled rulers, to subvert the government, or trample upon the rights of the people. The right of the citizens to keep and bear arms has justly been considered, as the palladium of the liberties of a republic; since it offers a strong moral check against the usurpation and arbitrary power of rulers; and will generally, even if these are successful in the first instance, enable the people to resist and triumph over them. And yet, though this truth would seem so clear, and the importance of a well regulated militia would seem so undeniable, it cannot be disguised, that among the American people there is a growing indifference to any system of militia discipline, and a strong disposition, from a sense of its burthens, to be rid of all regulations. How it is practicable to keep the people duly armed without some organization, it is difficult to see. There is certainly no small danger, that indifference may lead to disgust, and disgust to contempt; and thus gradually undermine all the protection intended by this clause of our national bill of rights." Justice Joseph Story
For More Information, see Sources on the Second Amendment and Rights to Keep and Bear Arms in State Constitutions