Summary
After the Americans defeated the British at the Battle
of Saratoga in 1777,  Franklin and his co-commissioners negotiated
with the French government for an  alliance. On January 28, they
reported that France had donated several million  livres–a hefty
sum of money–to the American cause. On February 6  they went even
farther, signing a treaty of alliance with France. This treaty 
was crucial. By itself America probably never could have defeated
Britain, but  with France's financial and military help, it would
only be a matter of time  before Britain gave up.
This moment came in 1781. After being defeated by a combined French
and American  force at the Battle of Yorktown, the British agreed
to negotiate a peace treaty.  Franklin and four others (including
John Adams and Thomas Jefferson) were appointed by Congress to negotiate
the  peace. Franklin met with Richard Oswald, Britain's negotiator,
from March to  June 1782. At one point Franklin nearly convinced
Oswald to agree to give Canada  to the newly independent colonies.
This deal fell through, however, when John  Jay demanded that Britain
recognize America's independence right away.
Oswald and Franklin finally reached a deal in July of
1782. After months of  further tinkering, the Americans signed the
Treaty of Paris on September 3,  1783. This officially ended the
Revolutionary War. After the treaty was formally  ratified on May
12, 1784, Franklin asked to be sent home. He got permission in 
May 1785 and sailed home two months later. He was now seventy-nine,
suffering  from a bladder stone and gout, and eager to be free of
politics once and for  all. He was already older than just about
any American alive, let alone any  American as busy as he.
Franklin arrived home in Philadelphia on September fourteen, 1785.
Within a  month, he was elected president of Pennsylvania's supreme
executive council, the  top government office. He held the job for
three years. Meanwhile, he continued  working on his Autobiography and
invented more useful things, such as a tool to take books from high
shelves and a desk chair  with a writing arm.
From May 28 to September 17, 1787, Franklin attended the Constitutional
 Convention. There, he played a key role in hammering out the "Great
 Compromise" that led to the signing of the Constitution. Along
with George  Washington, Franklin brought a cool head and immense
prestige to the meeting.  Without his help it would probably have
failed.
Even after retiring from politics altogether, Franklin
stayed active. In 1789  and 1790 he petitioned the new federal government to
abolish slavery and wrote a  brilliant satire of pro-slavery arguments.
He spent his last days fighting for  abolition, the last and perhaps
most passionate of his many causes, before  failing ill with pleurisy.
He died in his home on April 17, 1790, and was  publicly mourned
on both sides of the Atlantic as one of the greatest men of his
 era.