By mid-1781, when the military and naval crisis began which culminated in the fighting at Yorktown in October, the War of the American Rebellion had been going on for six years. Beginning with the skirmishes at Bunker Hill, at Concord, and at Lexington in Massachusetts, and the siege of the British forces in Boston in 1775, the area involved in the war had repeatedly widened, first throughout British North America, including Canada, then into the waters of the Atlantic Ocean and the Caribbean Sea, then in 1778 to Europe. France had been keen to encourage the rebels from the very first, in part to gain revenge for its defeat in the previous bout of the Anglo-French contest which had been a fixture in European affairs since the 1670s, but also in the hope that pieces of British imperial territory could be acquired. French arms and supplies had been shipped to the rebels, clandestinely at first, and then openly, and then France had joined in militarily. Spain was persuaded and encouraged to participate by an alliance with France, the Family Compact, agreed in 1779; the next year the British forced the Dutch to fight by declaring war, after what they considered to be years of considerable provocation. Each new participant extended the war to new geographical areas, not just in Europe but overseas, and ever more distantly from the original seat of fighting in North America. The French sent forces to the Indian Ocean, and the Dutch territory in South Africa became involved, as did the Dutch lands and islands in India, Indonesia, and the West Indies.

And, as more countries joined in, and more areas were affected, so the original purpose of the fighting became overlaid by other, more immediate, priorities. For the British, the War of the American Rebellion widened into a war, not just for part of the empire, but at times for its very survival. The Spanish involvement was, for Britain, particularly serious, since it brought into action a large, though less than efficient, navy which was able to join with that of the French, so that together they considerably outnumbered the Royal Navy; in addition the Spanish participation involved major sieges of Gibraltar and Minorca, and a Spanish campaign in Florida. In 1779 a serious attempt to invade the British Isles was mounted by a joint Franco- Spanish fleet and army.