How important was George Washington’s network of spies to winning the American Revolution?

It’s hard to imagine that the fate of the American cause would rest so heavily in the hands of a tailor, an enslaved double agent—or a judge’s wife who sent surreptitious signals on her laundry line. But as General Washington struggled to win a war with an army that was perpetually undermanned, undertrained and undersupplied, he relied increasingly on his unseen weapon: a secret intelligence network. Throughout the war, Washington’s spies helped him make bold, canny decisions that would turn the tide of the conflict—and in some instances, even save his life.

The story of Washington’s underground spy network, and how it helped Americans win their revolution, is replete with intrigue: letters written in invisible ink; a rare female agent who went by the mysterious moniker Agent 355; the gruesome execution of the spy Nathan Hale. Indeed, according to the Central Intelligence Agency, “General Washington was more deeply involved in intelligence operations than any American general-in-chief until Dwight Eisenhower during World War II.”

From the beginning of Washington’s meteoric rise, intelligence gathering helped shape his military career. He first learned to use on-the-ground information from Native Americans and deserting French soldiers during the French and Indian War. Intelligence, he learned, could make the difference between victory or death.

So in 1775, when the Second Continental Congress chose Washington as commander in chief of the Continental armies, Washington appointed a soldier named Thomas Knowlton to organize the war’s first spy unit. The roughly 130-man group, known as “Knowlton’s Rangers,” played a key role in the 1776 battle of Harlem Heights in New York, scouting out the British advance guard. In the blazing musket fire of the skirmish that followed, Knowlton was killed, his place in history cemented. Even today, the seal of the U.S. Army intelligence service bears a “1776” stamp, in honor of his unit.

In November 1778, General Washington appointed Benjamin Tallmadge director of military intelligence and ordered him to construct a spy ring inside New York City, which was by this time occupied by the British—and would be for the duration of the war). Dubbed “the Culper Ring” at Washington’s suggestion—a riff on Culpeper County in his home state of Virginia—it included Tallmadge (alias: “John Bolton”) and his chief recruit and childhood friend, Abraham Woodhull (alias: Samuel Culper Sr.). Woodhull went underground in New York, returning to Tallmadge periodically with reliable info about British operations.

The spy ring grew to include Robert Townsend (alias: Culper Jr.), who posed as a loyalist newspaper columnist and hung out in cafes, hobnobbing with vain British officers who wanted publicity and were happy to share intel.

It also included Anna Strong, the spy believed to have gone by codename 355. Strong lived in Setauket on the Long Island coast. Her job was reportedly to relay signals to couriers smuggling intelligence through Long Island Sound to Tallmadge, stationed in Connecticut. Her manner of communication was ingenious: She hung out her laundry on a clothesline—in full view of British soldiers and also of boats moving through the Sound.

If Strong hung a black petticoat, that signaled that a message was ready to be picked up by a courier. She would then hang handkerchiefs—the number of which would correspond to a secret pickup spot. Another Culper Ring spy, Caleb Brewster, who commanded whale boats in Long Island Sound, watched for her signals so that he would know, literally, when the coast was clear—and where the message was to be found.

The Culper Ring provided key intelligence throughout the war. Among these spies’ coups: They uncovered the British plan to crash the nascent Continental economy by printing massive amounts of counterfeit currency. They revealed a British plan to ambush the French fleet as it arrived in Rhode Island to support the American cause. And in 1780, they helped unmask the war’s most infamous traitor—Benedict Arnold, a patriot who, in exchange for £20,000, agreed to surrender the crucial American garrison at West Point to the British through their top spy, John André. However, André was caught with the plan’s details hidden in his boot; while Arnold ultimately escaped to Britain, John André was not so lucky.

The Culper Ring used code names to hide the identities of operatives. Even Washington had one—Agent 711. As head of intelligence, Tallmadge created the Culper Code Book, which assigned ciphers to 763 names or words. The number 219 denoted “gun”; 223 meant “gold”; 701 meant “woman.”

The brother of founding father John Jay, named James, invented an even more novel strategy. He invented a chemical solution—often using acidic fluids like lime juice, milk or vinegar—that functioned as invisible ink. Messages could be written, literally, in between the lines of what would appear to be an innocuous note. When treated with heat (say, over a candle) the secret writing would emerge. Washington instructed his agents to use this “sympathetic stain.” Not only would the messages be less exposed to detection, he told his operatives, it would “relieve the fears of such persons as may be entrusted in its conveyance.”

One of the most prolific spies in New York City began his activities before the establishment of the Culper Ring: Hercules Mulligan, assisted by his enslaved manservant Cato. Mulligan ran a clothing emporium catering to wealthy New Yorkers, including many high-ranking British officers. Mulligan became so chummy with these British military men, he married one officer’s sister. But he secretly supported the revolution.

Before the war, he had boarded a tenant in his home, a British loyalist named Alexander Hamilton, who he converted to the rebellion. It was Hamilton—who became an aide de camp to Washington—who brought Mulligan into the secret society of spies. Mulligan culled intel from his British clients, who thought he was on their side. Then he would dispatch Cato to inform Hamilton.

Twice Mulligan helped to save George Washington’s life. One night in his shop, Mulligan was fitting a British soldier who needed a coat because he was on a mission within the next 48 hours to capture General Washington, as the British had learned of Washington’s whereabouts. Mulligan quickly dispatched Cato, who informed the general in time.

In February 1781, the British learned of Washington’s plans to travel to Rhode Island along the Connecticut shoreline, and ordered 300 soldiers aboard a ship to intercept him. One of the men tasked with loading provisions on the ship was Hugh Mulligan, Hercules’s brother. Hugh managed to advise Hercules, who again dispatched Cato. Washington rerouted—and survived.

After the war, Washington was so thankful, he continued to buy clothes from Mulligan’s emporium—even after becoming president. Mulligan kept a sign out front of his shop: “Clothier to Genl. Washington.”

The story of James Lafayette Armistead is even more unlikely. Armistead was an enslaved African-American man who served not just as a spy—but as a double agent. In 1781, posing as a runaway slave, Armistead infiltrated a British outpost in Virginia and became a trusted informant by advising them about local terrain. This put him in position to quietly gather critical intelligence.

Armistead began smuggling military intelligence to Marquis de Lafayette, head of the French forces aiding General Washington. Armistead ultimately fed the generals intel that the British were moving thousands of reinforcements to Yorktown. This enabled the colonial forces to set up a blockade around the Yorktown peninsula—key to winning the critical battle of Yorktown, which ended the war. Due to his spy work, Armistead ultimately won his freedom from slavery.

In the annals of the Revolutionary War, there are two instances in which high-profile spies were captured—one from each side. Early in the war, when Washington knew that the British were going to attempt to sack New York City, he called for a spy behind enemy lines. Benjamin Tallmadge chose Nathan Hale, a classmate at Yale before the war. Hale was in New York posing as a Dutch schoolteacher looking for work when the British captured the city. After his identity was discovered (how is a matter of debate), Hale, just 21, was hanged on September 22, 1776. His last words were, reportedly, “I only regret that I have but one life to lose for my country.”

The most high-profile British spy to be captured was Major John André, the conspirator who worked with Benedict Arnold and was seized with the help of the Culper Ring. André was a man of such rank and nobility that Washington was unnerved by the idea of executing him. Still, a captured spy had to be sentenced appropriately. André requested death by firing squad, but Washington refused him. Instead, he was hanged in front of an audience of American officials. André whispered to himself just before he dropped from the gallows: “It will be but a momentary pang.”