The Founding Fathers may have been idealistic about Enlightenment principles like “Life, Liberty and the pursuit of Happiness,” but they were deeply realistic about the chances of a scrappy, underfunded colonial army to defeat the wealthy and powerful British Empire.

To win the Revolution, America needed allies, but more importantly it needed manpower, munitions and a lot of money.

That’s how 70-year-old Benjamin Franklin—famed inventor, publisher and elder statesman of the Continental Congress—found himself sailing to France in October of 1776. Although a monarchy itself, France was America’s best hope for backing the colonists in their bid for freedom from Great Britain, France’s perennial rival.

Franklin spent the next nine years in Paris as America’s first foreign ambassador. Without Franklin’s celebrity status, savvy networking abilities and unapologetic Francophilia, the American Revolution would almost certainly have failed. Instead, Franklin convinced France to nearly bankrupt its own government in order to ensure American independence.

In accepting his mission to France, Franklin put his life on the line. Not only was the Atlantic crossing treacherous for all of the normal reasons—stormy seas, shipboard illnesses, piracy—but Franklin was also sailing as a traitor. By signing the Declaration of Independence just months earlier, Franklin would have been hanged if captured by the British Navy.

Franklin survived the arduous journey—his seventh trans-Atlantic crossing—and arrived in Paris as a bona fide 18th-century celebrity.

“Franklin was the most famous American in the world,” says biographer Stacy Schiff. “He was the discoverer of electricity, a man of genius, a successor to Newton and Galileo. He also counted among the greatest celebrities in Paris; he could not walk through the street without attracting a crowd.”

Franklin’s unmistakeable image—wearing glasses and a fur cap instead of a wig—was embossed on collectible candy dishes, stitched into clothing and engraved into snuff boxes and walking sticks.

While Franklin himself was famous, most French people knew nothing about the American Colonies or their politics. Even foreign ambassadors stationed in France had no idea why the famed scientist had come to Paris. For his health, maybe, or to ensure that his 16-year-old grandson William Temple Franklin received a proper European education? The Portuguese ambassador, Schiff reports, was sure that Franklin’s plan was to retire to a Swiss chateau with his vast wealth.

In reality, Franklin had only one reason for being in Paris—to convince the French to make a very expensive bet on America. But first, he had to let all of the political pieces fall into place. Or more accurately, Franklin had to use all of his unique talents to make sure that they fell into place.

For his first 18 months in Paris, Franklin was in a difficult position. The Continental Army suffered a string of defeats and it wasn’t at all clear that the Americans had a fighting chance. Versailles was quietly supportive of the Revolution, but the French crown couldn’t risk backing the wrong horse and getting ensnared in another losing war with England.

So Franklin did something that his younger American colleagues found incredibly frustrating—he played the waiting game. Now in his 70s, Franklin had little to prove and was in no hurry to prove it. Instead of banging on the door of the French foreign minister, the Count de Vergennes, Franklin attended salons with influential aristocrats and engaged in a very French, very laissez-faire charm offensive.

“Franklin was at all times a master psychologist,” says Schiff. “He read people, and cultures, easily. He quickly mastered the French art of accomplishing much while appearing to accomplish little.”

Franklin was also a confident bluffer. He insisted that Washington commanded an army of 80,000 men, when 14,000 was closer to the truth. Franklin laughed off every British victory, insisting that King George III was playing right into the Americans’ hands. The Patriots had superior firepower, fiercer fighters, and more than anything, an unquenchable thirst for freedom. The British would need an army of 200,000 to beat them.

None of this was true, of course. The Americans were dangerously low on supplies and outgunned in every battle. In private, Franklin was deeply anxious. For a year, Franklin had heard almost nothing from Congress, leaving him to fear the worst.

Then, on December 4, 1777, an American messenger arrived at Franklin’s estate with two pieces of news. First, the bad news: the British had occupied Philadelphia, the American capital, and Washington’s army had retreated to winter camp at Valley Forge. But there was also good news—fantastic news, even! In October, the Americans had dealt a stunning defeat to the British at the Battle of Saratoga, capturing General John Burgoyne and his men. (The hero on the American side was General Benedict Arnold.)

Franklin didn’t need to bluff anymore. Versailles was so impressed by the decisive victory at Saratoga that France signed treaties of alliance with the Americans on February 6, 1778.

As the lead American ambassador in France, Franklin had his hands full. His main duty was to solicit money, weapons, uniforms, ammunition and other critical supplies from the Count de Vergennes, which itself was a full-time job.

“At one point Franklin received a 38-page shopping list,” says Schiff. “It included a frigate, a ship of the line, and 49,000 uniforms—as well as spoons, trumpets, paint and thimbles. The list left Franklin speechless.”

On top of that, Franklin’s home in the village of Passy outside of Paris was continuously bombarded with visitors, each seeking an audience with the famous American ambassador.

According to French custom, Franklin always made time for guests—invited or not. Some were French industrialists hoping to sell shoes, blankets and beer to the American army. Others were eager to enlist and fight (if the rumors of free tracts of land in America were true). Inventors wanted Franklin’s opinion on their ideas for novel explosives or “fireproof wood,” and every American in France with a hard-luck story needed Franklin to arrange their free passage home.

To meet all of these demands without offending French social sensibilities, Franklin lived a type of double life. To outside observers, he was the quintessential cultured French gentleman, the type who arrived appropriately late for appointments and greeted unexpected guests with wine and unrushed conversation. But in private, Franklin put in 14-hour days, often waking in the middle of the night to finish piles of paperwork.

Franklin was so good at making hard work look easy that he fooled even his American colleagues into thinking he was more interested in flirting with French widows than laboring for the Revolution. And no one misunderstood and resented Franklin’s tactics more than John Adams.

John Adams arrived in France in 1778 to replace Silas Deane, an American ambassador dismissed for fraud. Adams was a brilliant writer and political philosopher, but his blunt, straight-talking demeanor clashed with French courtly manners. Instead of easing Franklin’s load in Paris, Adams’s very presence became an obstacle.

“The two men got off on the wrong foot and remained there,” says Schiff. “It didn’t help that Adams failed to ingratiate himself at Court and resented Franklin’s tremendous celebrity.”

In letters home, Adams complained bitterly about Franklin—everything from the older statesman’s inferior French to the way the famous inventor was greeted like “an opera girl” everywhere he went.

As foreign ambassadors, Adams and Franklin couldn’t have been more different in style and personality. Adams refused to acknowledge America’s indebtedness to France and approached Versailles with urgent ultimatums for more supplies and military support. In contrast, Franklin was ingratiating and patient—always careful to make requests of America’s benefactors, not demands.

By the end of the war, Adams and Franklin still disliked each other immensely, but they were able to see past their differences long enough to successfully negotiate a peace treaty with Britain that recognized America’s independence. Through it all, Adams’s opinion of Franklin never improved.

“If I was in Congress, and this gentleman and the marble Mercury in the garden at Versailles were in nomination for an embassy, I would not hesitate to give my vote for the statue,” Adams wrote a colleague in 1783, “upon the principle that it would do no harm.”

America could not have won the Revolutionary War without France. Schiff estimates the total value of French material and manpower at roughly $20 billion in today’s money. It was enough to bankrupt the government of Louis XVI, one of the aggravating factors that led to the French Revolution.

“When the British surrendered at Yorktown they did so to forces that were nearly equal parts French and American,” says Schiff, “all fed, clothed and paid by France, and protected by a French navy.”

Benjamin Franklin was the reason why France opened its coffers so wide to the unproven Americans. To put it simply, the French liked him and trusted him.

“Nothing could have been more critical to our Revolution than that affection,” says Schiff. “Every other American envoy who approached Versailles bungled along the way. Franklin was inventing the foreign service out of whole cloth. And he was, as we know from so many other realms, a brilliant inventor.”

Franklin was almost 80 when he crossed the Atlantic a final time and returned to a Philadelphia he hardly recognized. America had changed immensely in the nine years he was laboring abroad, and that included a new generation of politicians. Franklin had hoped to receive some compensation for his difficult mission—as others had—but Congress didn’t want to dwell on the debt America owed to France.

“The French mission had been, hands down, the most taxing assignment of Franklin’s life,” says Schiff. “Congress never offered a settling of accounts, a reward, or so much as a single syllable of thanks.”