Queries about "Up and at 'em"

[Cohen, Gerald Leonard]

   Would anyone know when the expression "Up and at 'em" is first attested? Also, I assume the original context was a battle, where an officer tells his men in effect: "Get up  (from a defensive position) and charge at them (the enemy)."  Is there any reason to doubt this assumption?

     These queries were suggested to me by a student.

Gerald Cohen

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