The event that signaled the beginning of the American Revolutionary War was the Battles of Lexington and Concord, which occurred on April 19, 1775. These skirmishes marked the first military engagements between British troops and colonial militias. The tensions had been escalating due to issues such as taxation without representation and British attempts to assert control over the colonies. The battles not only demonstrated the colonists' willingness to fight for their rights but also galvanized support for the revolutionary cause, leading to a full-scale war for independence and ultimately the establishment of the United States.