Wherever water routes could be used the naval power of the British gave them an advantage.

One such route was the Hudson, less a river than a navigable arm of the sea, leading to the heart of the colony of New York, its upper waters almost touching Lake George and Lake Champlain, which in turn led to the St.Lawrence in Canada and thence to the sea.

Canada was held by the British; and it was clear that, if they should take the city of New York, they might command the whole line from the mouth of the Hudson to the St.Lawrence, and so cut off New England from the other colonies and overcome a divided enemy.

To foil this policy Washington planned to hold New York and to capture Canada.

With Canada in line the union of the colonies would be indeed continental, and, if the British were driven from Boston, they would have no secure foothold in North America.
The danger from Canada had always been a source of anxiety to the English colonies.