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.

Not to draw too fine a point on it, some of the founders did not want the government to maintain a standing army because of the cost and inconvenience (the possibility of mischief was also discussed). And some of the founders feared that the government might institute a standing army that would be employed to among other things replace the militias--a step toward tyranny.

But the ability of communities in the far-flung states to defend themselves from attack by marauders or to suppress local rebellion (in the South, much-feared slave uprisings) required some kind of trained, armed group. A militia. The notion was that an army could quickly be raised by calling the militias into national service in the event of a major rebellion (as had been done to put down the recent Shays' tax rebellion in Massachusetts) or foreign invasion.

Your basic Constitution does provide for raising an army when needed, and for oversight of the state militias. The Second Amendment is about what the federal government could not do--interfere with the states' necessity to maintain militias: white males trained to bear arms in defense of the community.

But by the mid-1790s the defense of the frontier, the suppression of rebellions and resistance to land invasion became the province of a United States Army and has been more or less ever since. The militias and their need for Second Amendment protection shrivelled. The notion that the amendment provides protection for individuals' gun ownership is a recent interpretation that serves both the paranoid and the gun industry.

(Dave Sagarin, April 10, 2013; slightly modified February 20, 2018 )

NOTES: there are a number of remarks attributed to Thos Jefferson about the beauty of the Second Amendment and the need for the Federal Government to fear the people, etc. They are spurious.

Before the writing of the Constitution, a number of the colonies had already thought about the issue--Here for instance is Section 13 of the Virginia Declaration of Rights, 1776.

That a well-regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defense of a free state; that standing armies, in time of peace, should be avoided as dangerous to liberty; and that in all cases the military should be under strict subordination to, and governed by, the civil power.

And here's the relevant section in the U.S. Constitution, Article 1 Section 8, which includes the following:

The Congress shall have Power

• To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;
• To provide and maintain a Navy;
• To make Rules for the Government and Regulation of the land and naval Forces;
• To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
• To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;