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Extra credit: under which branch of government does the Constitutional prescription for the security of a free state fall?

Um, it doesn't fall under any branch of government. According to the Second Amendment, the responsibility for the security of a free state falls to a well-regulated militia, which is composed of the people, not the government. Defense of freedom isn't a government function at all. As a matter of fact, given that the government is one of the chief threats to liberty against which the Founding Fathers wanted to guard, isn't it a conflict of interest, on the order of hiring the fox to guard the henhouse, to delegate the defense of freedom to it?

"But whether the Constitution really be one thing, or another, this much is certain--that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist." --Lysander Spooner, 1867