(Definition of harp on (about sth) from the Cambridge Advanced Learners Dictionary & Thesaurus Â© Cambridge University Press).
Battle of Saratoga. In late September and during the first week of October 1777, Gate's American army was positioned between Burgoyne's army and Albany. On October 7, Burgoyne took the offensive. The troops crashed together south of the town of Saratoga, and Burgoyne's army was broken. In mop-up operations 86 percent of Burgoyne's command was captured.
The American victory Saratoga would prove to be a turning point of the American Revolution, as it prompted France (which had been secretly aiding the rebels since 1776) to enter the war openly on the American side, though it would not formally declare war on Great Britain until June 1778.