Which battle began with the shot heard round the world?

The "shot heard round the world" is a phrase from Ralph Waldo Emerson's "Concord Hymn" written in 1837:

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.

The poem refers to the beginning of the American Revolutionary War and the "shot heard round the world," was at the Battles of Lexington and Concord, considered to be the first open conflict of the war. The shots fired there were the beginning of a war that would so drastically change the future of the world (with the eventual creation of the United States), that the world could almost hear it.