The decade 1775-1785 is a moment in American history that Americans know well, or think they do. These years enclose the brave event that we have been celebrating, one after another, in this bicentenary decade. But along with the exploits at home, there was a diplomatic effort going on in the courts of old Europe.

Susan Mary Alsop's Yankees at the Court tells the story of this first decade of American diplomacy. It does the job neatly, giving us a picture of events that were often decidedly murky.

Benjamin Franklin, John Adams, John Jay and others went to Europe to elicit aid in the 13 colonies' struggle with Britain. They succeeded in France, largely because the French foreign minister, Vergennes, wanted to redress the balance of power between France and England by detaching England's North American colonies from her. Alsop is unequivocal about how we won the Revolutionary War: "As it turned out, it was to be France that would save the United States from defeat at the hands of Great Britain. Unquestionably, the war of Independence would have been lost by 1778 without French aid."

The diplomatic story begins with a shadowy figure, the French agent Bonvouloir, sent to Philadelphia to sound out the Americans' determination to resist the British. He met with Franklin's Committee of Secret Correspondence and reported back to France that Americans meant to be free. It was this humble mission that launched our diplomatic history. The French decided to send guns.

Silas Deane, a Connecticut merchant, went to France to organize the shipments, and found himself dealing with another secret agent, the celebrated Beaumarchais. What an ill-assorted pair! Beaumarchais, the ultimate in worldliness; Deane a tidy lonely democrat. They had no language in common. Yet they managed between them to fill a fleet of ships with guns for the war of independence. Spies were there to report it all to the British navy, and half the privateers never made it. But enough got through to give Washington a victory at Saratoga. Alsop portrays this odd couple, and their unsung triumph, with due regard for the documents.

Franklin arrived in Paris and the secret French subsidies grew. But how secret were they? Enter Dr. Edward Bancroft, smiling, comforting, the closest friend of the American mission in Paris. He shared their political problems for eight years, and went once a week to a hollow tree in the Tuileries gardens to deliver all their secrets up to British agents.

And now Vergennes presented Louis XVI with his options: abandon the Americans or give them overt help. This was a pivotal decision. Franklin, in one of his most adroit moves, entertained a known British spy for hours, in full view of the French, to increase French anxiety over an Anglo-American rapprochement. Could this be what finally pushed the French into an open treaty with the Americans?

It made the British rush to America with new peace terms, but the Franco-American treaty reached the Continental Congress first. Congress decided in favor of complete independence.

The American mission was plagued with dissension, and here Alsop sets out unpleasant facts with care. Arthur Lee, who quarreled with everyone and wanted Franklin's job, convinced Congress that French aid was not a loan but a gift. This resulted in a global misunderstanding. Beaumarchais waited in vain for the ships to return from America laden with produce. He and others who had helped finance the venture lost a fortune. America never paid the French back.

Adams came to Paris and was lionized, all the while decrying the opulence in which Franklin seemed to move serenely. John Jay, after two humiliating years in Madrid, arrived in Paris in turn, to join in the peace negotiations.

These green plenipotentiaries had clear instructions from Congress: "Be governed by the advice and opinion of the king of France." But Congress was too far away to see the forces at work in the European power game. The Americans at Versailles divined that Vergennes did not want a strong America but a weakened England. Therefore the American diplomatists made an agonizing choice: they violated their instructions and dealt with the British on their own.

And the British found them surprisingly equal to the task. Alsop recounts the shrewd maneuvers of Jay, refusing to be "cooped up within the Alleghenies," and rejecting the use of the word "colonies" in the treaty.

At last a treaty was signed by an independent America with the British. Vergennes, who had given so much in money and troops to winning the war, now had to be told. Franklin took on this disagreeable task, and did his best to smooth ruffled French feelings.

Apart from its honesty in helping to correct the record of French involvement in our revolution, this book is rich in vignettes of the first Americans in Paris: Abigail Adams, primly scandalized by the loose manners of Mme. Helvetius. John Jay, putting on the new £30 suit offered him by Congress to wear to Versailles.

Alsop writes with sharp details and clear images. Because she avoids the gratuitously witty phrase in favor of accurate description and analysis, we feel we can trust her historical sense.