"Opposition to the use of military force in the enforcement of civil law is deeply imbedded in American tradition. It derives both from British precedents and from the experiences of the American Revolution, the ostensible cause of which was the use of British troops to enforce oppressive measures. The image of hated Redcoats shooting down innocent citizens in the Boston Massacre of 1771 was a vivid one, easily transferable to any soldier employed as an instrument of internal control by a central government. The sentiment that a standing army in time of peace was 'dangerous to the liberties of the people,' a standard article of faith of almost all Americans in the post-Revolutionary period, derived largely from fears that such an army would be used by an arbitrary government to tyrannize its own people, not to fight its foreign foes . A standing army, it was thought, could be the instrument only of a monarchy, not of a democratic state."
Center for Military History: http://www.history.army.mil/