Final answer:
Minutemen were waiting for the British in Lexington due to forewarning about their approach and their readiness to engage in military action at a moment's notice, which led to the first battle of the American Revolutionary War.
Step-by-step explanation:
There were Minutemen waiting for the British troops when they arrived in Lexington because they had been rallied following a warning system consisting of riders like Paul Revere and William Dawes, along with other signals such as bonfires and bells. These Minutemen were part of the colonial militia, which was prepared to respond at a moment's notice to any military threat, in this case, the movement of British troops out of Boston towards Lexington and Concord. On April 14, 1775, British General Thomas Gage dispatched 1,000 troops to these towns with knowledge of the militias and an intent to quash any potential rebellion. The Minutemen, assembled after hearing about the approaching British, were positioned on the Lexington Green, anticipating the encounter that would lead to the first battle of the American Revolutionary War.