The Battle of Ushant in 1778, rather than being indecisive, turns into a major French victory, with the British suffering significant losses. Alternatively, a major storm devastates the British fleet in the Channel. Either way, the Royal Navy finds itself more seriously outnumbered.
The resulting French naval superiority in the Channel means they don't need to combine with the Spanish fleet, and thus the Armada of 1779 turns into a successful invasion of the British Isles. As the French army approaches London, the British are forced to make peace, part of which includes (as per the Franco-American Alliance) the independence of the United States.