Final answer:
Ethan Allen and Benedict Arnold captured Fort Ticonderoga in May 1775, securing artillery that was essential in forcing the British evacuation from Boston in March 1776.
Step-by-step explanation:
The fort that Ethan Allen and Benedict Arnold captured to provide artillery for the Continental Army, which was subsequently used to drive the British out of Boston, was Fort Ticonderoga. This strategic move happened in May 1775, shortly after the battles of Lexington and Concord.
The capture of Fort Ticonderoga by Ethan Allen, Colonel Benedict Arnold, and the Green Mountain Boys provided the Continental Army with a significant boost in firepower. The cannons obtained from the fort were transported and positioned on the hills overlooking Boston, which played a pivotal role in ending the nearly year-long siege.
Unable to return fire effectively due to the elevation and placement of the colonial cannons, the British recognized their disadvantaged position and evacuated their troops to Halifax, Nova Scotia, on March 17, 1776.