The actual, historic situation in May, 1778
In May 1778 and the War for American Independence was in its fourth year. The main British Army, now commanded by General Sir Henry Clinton, was preparing to leave Philadelphia and march North to join the British garrison in that city. The main American army, commanded by General George Washington, passed an unpleasant winter at Valley Forge, but now benefited from the new, and most welcome French alliance. After the American victory at Saratoga, and the surrender of Burgoyne's army in the fall of 1777, France came into the war and immediately began sending badly needed supplies and equipment to the Americans. Washington's men were no longer a so-called "rabble in arms", but were experienced, battle-hardened soldiers who, though still not trained to the level of the British Regulars, were far from the woefully inept amateurs of two years earlier. And the British, though still confident of their superiority, knew this.
In the fall of 1776, while General Sir William Howe still commanded all British forces in North America, then Lieutenant General Henry Clinton was sent North from New York to capture Newport, Rhode Island. This provided the British with a seaport close to Boston and posed a constant threat to all of New England. By May of 1778 the British forces in Newport were commanded by Major General Lord Robert Pigot. His three thousand men were watched - but not particularly bothered - by the one thousand man American force commanded by Major General John Sullivan, headquartered near Tiverton. The closest other American force was in and around Boston, which was defended by strong artillery and a sizeable militia force. Although General Sullivan had some good Continental Line troops, and Light Infantry, the elite of the American forces in 1778, plus decent artillery, his main force was comprised of local militia units. Most Continentals were serving with Washington, so the New England colonies were largely on their own.
The It-Never-Happened-But-It-Could-Have Situation, May 21, 1778
General Pigot has learned that a large amount of newly arrived supplies and equipment provided by French ships has reached Boston, and is currently being moved Southwest toward the Hudson Highlands in New York, where they will greatly aid the American forces guarding the Hudson River North of New York City. The Americans, according to Loyalist spies, are moving these supplies slowly and carefully, with guards provided by the various towns as the caravan moves, along with an assigned force of Continentals and some artillery. General Pigot sees an opportunity and believes that a fast moving raiding force might be able to find, capture, and destroy these supplies. Pigot has selected an experienced officer to lead the raid. His choice, Brigadier General in America Kenneth Siegel, is from a military family that came to England along with King George the First, when that Hanoverian ascended to the English throne in 1714. A veteran of the French and Indian War, Siegel has many years of experience in North America and in fact served with his opponent, the commanding officer of the Supply caravan's guards, Brigadier General Bradford Chetwynd, during several campaigns in 1759 through 1763. The two were, in fact, friends but now they are enemies. Both men are seasoned soldiers who have fought well and hard since the very beginning of the war. Although each would regret it, they will not shrink from doing that which may be necessary even though it might result in the death of the other. Neither knows that the other is, in fact, the 'enemy commander'.
The same Loyalist spies that informed General Pigot of the supply movement have kept General Siegel informed of the location each day as the caravan moved. Siegel knows that the supplies arrived in the town of Needham on May 1st, and that they are still there because General Chetwynd has not yet gathered the necessary draft animals he needs to continue. Too many horses and oxen have died along the way, and the local farmers are reluctant to part with their animals because they need them for plowing and planting. Chetwynd has established an armed and well defended encampment just outside the town of Needham where he waits with growing impatience and concern for the needed draft animals to be collected. He also has received his own reports that a British force is moving somewhere to his South, but not exactly where. Although the Americans are prepared to move out of their camp and resume the march quickly, they have, as soldiers always will, turned what had been a temporary marching camp into more of a garrison fort. Many Continentals and Militia have their families with them in the camp. This constantly provides General Chetwynd with discipline problems which he could very well do without.
It is May 21st, 1778, and very soon the residents of the town of Needham will indeed know that the British are Coming!