Better and more interestng than the Jesus explanation.I think the song is about Herod the Great himself : http://en.wikipedia.org/wiki/Herod_the_Great
Herod used to rule the world : he was a very successful King.
Then in the morning he slept alone : afraid by the plots, he ordered to kill his wife. Once she was gone, we can suppose that for Herod there was never an honest word.
The Roman made him King of Israel, for them. So "he heared Jerusalem bells ringing", calling for him, and at the same time, "Roman cavalry choirs were singing Be my mirror, my sword and shield, My missionaries in a foreign field."
No ? Well that's my interpretation, do what you want with it !