Arguably, the most important contribution Benjamin Franklin made to United States history was acting as an ambassador and emissary to France during the Revolutionary War. Franklin's diplomacy, charming personality and good humor assured the Colonies of France's assistance against England, without which the fledgling nation would likely have lost their war for independence.
Franklin was also critical in maintaining American freedom after the surrender of Great Britain. France wanted to replace Britain as the United States' benefactor and tried to bully the new country into tying the American peace treaty with Britain to the French one. Had this happened, America could have been dragged into the French quarrels with the British. Franklin, who knew the French politicians and diplomats well, was prepared for their maneuvers and ensured the new government avoided these pitfalls. As part of this, Franklin personally secured a separate treaty for the United States with the British secretly.
Besides this, Franklin served as Postmaster General for the United States and is credited as being the father of the U.S. Postal Service. He helped draft the Declaration of Independence and was the oldest signer of that document. For years, he directed philanthropic work to support causes like public education and libraries. At the end of his life, he encouraged Congress to abolish slavery, using the power of his position as statesman as well as his office as the first president of the Pennsylvania Society for the Abolition of Slavery.