“The militia is the natural defense 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.”