The North Bridge, often colloquially called the Old North Bridge, is a historic site in Concord, Massachusetts spanning the Concord River. On April 19, 1775, the first day of the American Revolutionary War, provincial minutemen and militia companies numbering approximately 400 engaged roughly 90 British Army troops at this location. The battle was the first instance in which American forces advanced in formation on the British regulars, inflicted casualties, and routed their opponents. It was a pivotal moment in the Battles of Lexington and Concord and in American history. The significance of the historic events at the North Bridge inspired Ralph Waldo Emerson to refer to the moment as the "shot heard round the world."