From 1778, the British army and navy were additionally overstretched in a global war against France, and later against Spain and Holland. From Blenheim in 1704 to Waterloo in 1815, Britain won the majority of its victories in alliance with other countries in Europe.... In the American war, on the other hand, Britain was at a great disadvantage because it had no allies and was opposed by much of the rest of Europe in the League of Armed Neutrality (1780). Spain and France were able to concentrate upon building up their navies whose combined strength outnumbered the Royal Navy and Britain faced the most serious invasion threat since the Spanish Armada in 1588.... In consequence of the expansion of the war, the priorities of the British government were diverted from the war in America.
The Men Who Lost America: British Leadership, the American Revolution, and the Fate of the Empire (2013)