Read Federalist #46, written by James Madison, the original author of the Constitution, and later the Bill of Rights. You'll see that he thought it perfectly proper for the militia to protect the people of the several states from a federal standing army.
The highest number to which, according to the best computation, a standing army can be carried in any country, does not exceed one hundredth part of the whole number of souls; or one twenty-fifth part of the number able to bear arms. This proportion would not yield, in the United States, an army of more than twenty-five or thirty thousand men. To these would be opposed a militia amounting to near half a million of citizens with arms in their hands, officered by men chosen from among themselves, fighting for their common liberties, and united and conducted by governments possessing their affections and confidence. It may well be doubted, whether a militia thus circumstanced could ever be conquered by such a proportion of regular troops. Those who are best acquainted with the last successful resistance of this country against the British arms, will be most inclined to deny the possibility of it. Besides the advantage of being armed, which the Americans possess over the people of almost every other nation, the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition, more insurmountable than any which a simple government of any form can admit of. Notwithstanding the military establishments in the several kingdoms of Europe, which are carried as far as the public resources will bear, the governments are afraid to trust the people with arms.
Then there is opinion of Elbridge Gerry on the matter:
What, Sir, is the use of a militia? It is to prevent the establishment of a standing army, the bane of liberty. . . .Whenever Governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise an army upon their ruins."
- Rep. Elbridge Gerry of Massachusetts, Signer of the Declaration of Independence, VP of the United States 1813-1814, spoken during floor debate over the Second Amendment, I Annals of Congress at 750, August 17, 1789
I would add that the "standing army" of today that is a real threat to our liberty is not the US Military, they are pretty apolitical and most take their oath to "support and defend" pretty seriously, but rather the swarms of federal officials, including federal police, such as the FBI, BATFE, etc, etc, that are even now eating out our substance.