His idea was to get succors from France for this especial purpose, and with them and American aid to achieve the conquest.
Congress was impressed and pleased by the scheme, and sent a report upon it to Franklin, to communicate to the French court, but Washington, when he heard of the plan, took a very different view.
He sent at once a long dispatch to Congress, urging every possible objection to the proposed campaign, on the ground of its utter impracticability, and with this official letter, which was necessarily confined to the military side of the question, went another addressed to President Laurens personally, which contained the deeper reasons of his opposition.

He said that there was an objection not touched upon in his public letter, which was absolutely insurmountable.

This was the introduction of French troops into Canada to take possession of the capital, in the midst of a people of their own race and religion, and but recently severed from them.
He pointed out the enormous advantages which would accrue to France from the possession of Canada, such as independent posts, control of the Indians, and the Newfoundland trade.

"France, ...

possessed of New Orleans on our right, Canada on our left, and seconded by the numerous tribes of Indians in our rear, ...