The reason why controlling the region of Boston, New York, and Philadelphia was important to both sides of the war was because it could allow for reinforcements by water.
During the Civil War, a lot of significant battles took place around Boston, New York, and Philadelphia as both the British and the American colonists tried to hold onto these cities.
The reason for this was because of the ports that were available in each of those cities. These ports meant that the side holding it could get supplies from outside that would enable them to keep fighting, and even win the war.
For the British, these cities were especially important as they were relying significantly on supplies from Europe to maintain the war effort.
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