Here is the text of the Second Amendment to the U.S. Constitution as ratified by the States:
"A well regulated militia being necessary to the security of a free State, the right of the People to keep and bear arms shall not be infringed."
Some people believe that the Second Amendment is about the right to defend oneself from physical attack, and in a related sense, about protecting one's family and property from criminal attack or theft. But that is only partly what the Amendment is about. You certainly do have the right to defend yourself, your loved ones, and to protect your property, but if the Second Amendment were only about that, it would be lacking an essential ingredient of liberty.
The Second Amendment is actually about something more important than personal defense. A hint of what it is comes from this passage in the Declaration of Independence:
"[W]henever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government."
Had the original Colonists been disarmed by the British government, there could have been no Revolution, and there would have been no independent United States of America. The Second Amendment is primarily about preserving the possibility that if the national government becomes tyrannical, and if, after exhausting all peaceful means available to them, the people are unable to abate the tyranny, the people have the right, and by the 2nd amendment cannot be denied the practical means, of forcibly replacing an otherwise unchangeable tyrannical national government.
Many people doubt this aspect of the Second Amendment, but there is ample evidence that it is about preserving the concept that the government may only govern with the consent of the governed:
Americans have the right and advantage of being armed - unlike the citizens of other countries whose governments are afraid to trust the people with arms.
-- James Madison, The Federalist Papers
"The best we can hope for concerning the people at large is that they be properly armed."
-- Alexander Hamilton, The Federalist Papers at 184-188
"One of the ordinary modes, by which tyrants accomplish their purposes without resistance, is, by disarming the people, and making it an offense to keep arms."
-- Constitutional scholar and Supreme Court Justice Joseph Story, 1840
"The bearing of arms is the essential medium through which the individual asserts both his social power and his participation in politics as a responsible moral being..."
-- J.G.A. Pocock, describing the beliefs of the founders of the U.S.
Men trained in arms from their infancy, and animated by the love of liberty, will afford neither a cheap or easy conquest.
-- From the Declaration of the Continental Congress, July 1775.
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 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."
-- Supreme Court Justice Joseph Story of the John Marshall Court
Militias, when properly formed, are in fact the people themselves and include all men capable of bearing arms. [...] To preserve liberty it is essential that the whole body of the people always possess arms and be taught alike, especially when young, how to use them.
-- Senator Richard Henry Lee, 1788, on "militia" in the 2nd Amendment
That the said Constitution shall never be construed to authorize Congress to infringe the just liberty of the press or the rights of conscience; or to prevent the people of the United states who are peaceable citizens from keeping their own arms...
-- Samuel Adams, in "Phila. Independent Gazetteer", August 20, 1789
Incidentally, you have the right to bear arms whether or not the U.S. Constitution (or your national government's laws) explicitly say so. This is a right with which every human being is born. Your government may, of course, without moral validity, forbid you to be armed, which is another matter entirely.
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