In OTL, we are aware that Washington wanted to storm Boston in 1776 but was talked out of it, acquiring the city peacefully, but not taking its garrison or all that much supplies, and these troops and supplies would later be used against Washington in New York. Obviously, a victory in Boston that takes out the garrison to a man and seizes a ton of war material would be a huge boon to the American cause.
How does this result come about?
I think one way that it MIGHT have a chance is if the Salem and Marblehead Militia, under Timothy Pickering, cuts off the British retreat from Concord by taking the bridge that Percy needed to get back to Charlestown. Historically, Pickering did not take the bridge and block the retreat despite arriving on the field in time and position to do so, and was accused of not doing so because he held a wish for the conflict to be resolved peacefully. This would likely make the entire expedition fall prisoner to the pursuing forces under Heath and Warren, almost 1500-2000 men total, and leave the garrison in Boston critically short handed with all of its flank companies, the best forces they had, in the hands of the Americans.
At this point, I think an assault might be possible, as there would likely be under 3000 combat ready men in the city remaining, facing upwards of 19000 American militia, albeit disorganized troops, and the British would be without a good portion of their cannon and all of their elite troops. Once Howe and Clinton arrive with reinforcements, the chances of success go down dramatically.
Any thoughts?