Shot heard around the world?

I'm looking for the origin of the phrase, 'The shot heard around the world'. I tend to remember it as being the reason
for the start of WW1. A shooting involving some Duke or King. Any ideas?

Emerson.
By the rude bridge that arched the flood,
Their flag to April's breeze unfurled,
Here once the embattled farmers stood,
And fired the shot heard round the world.

Of course, it has been applied ever since to any number of other significant "shots" including various assassinations, and one highly disappointing home run in a 1951 baseball playoff, but I'd rather not relive that painful memory.