The Army of the Potomac was an army for the Union.
Prior to the English Civil War in England, the monarch maintained a personal bodyguard of Yeomen of the Guard and the Honourable Corps of Gentlemen at Arms, or "gentlemen pensioners", and a few locally raised companies to garrison important places such as Berwick on Tweed or Portsmouth (or Calais before it was recaptured by France in 1558)
 The defense of the state now rested on the commoners, not on the aristocrats

