Rochambeau (this was his title, not his given name--he was the comte de Rochambeau) played a major part in winning the Revolutionary War and thus helping the US become a democracy.
Rochambeau came to America with an army of over 5,000 soldiers to help the colonists fight aganst the British. This was in 1780. Rochambeau helped George Washington decide on and then execute a plan to fight against the army that was under Lord Cornwallis in Virginia. Rochambeau and Washington led their combined army against Cornwallis in what has come to be known as the Battle of Yorktown. They defeated the British, thus leading to the end of the war.
By helping the colonists win the Revolution, Rochambeau helped the US become a democracy.