They were, ahem, "a well regulated militia" -- regulated by the Massachusetts Provincial Congress -- who stored militia weapons & ammunition in locked buildings in small towns like Concord.
But in February 1775 the British Parliament declared them to be in rebellion -- effectively a declaration of war -- and British forces began moving to suppress the rebellion.
Lexington & Concord were the first major clashes of American militia against British regulars.
Interestingly, Paul Revere rode to Portsmouth NH in December of 1774, long before his ride into the countryside of Massachusetts. His visit to New Hampshire was also to warn of plans by the British.
This article has more details of a fascinating and often ignored early action against the British.
Theres a pending perverse parallel brewing between then and now.