The battle was fought in the county of Northumberland in northern England on 9 September 1513, between an invading Scots army under King James IV and an English army commanded by the Earl of Surrey. It was a decisive English victory. In terms of troop numbers, it was the largest battle fought between the two Kingdoms.
James IV was killed in the battle, becoming the last monarch from the British Isles to suffer such a death.
This conflict began when James IV, King of Scots declared war on England to honour the Auld Alliance with France by diverting Henry VIII's English troops from their campaign against the French king Louis XII. Henry VIII had also opened old wounds by claiming to be the overlord of Scotland, which angered the Scots and their King. At this time England was involved as a member of the "Catholic League" in the War of the League of Cambrai—defending Italy and the Pope from the French (see Italian Wars).
Pope Leo X, already a signatory to the anti-French Treaty of Mechlin, sent a letter to James threatening him with ecclesiastical censure for breaking his peace treaties with England on 28 June 1513, and subsequently James was excommunicated by Cardinal Christopher Bainbridge. James also summoned sailors and sent the Scottish navy, including the Great Michael, to join the ships of Louis XII of France.
Henry was in France with the Emperor Maximilian at the siege of Thérouanne. The Scottish Lyon King of Arms brought James IV's letter of 26 July to him. James asked him to desist from attacking France in breach of their treaty. Henry's exchange with Islay Herald or the Lyon King on 11 August at his tent at the siege was recorded. The Herald declared that Henry should abandon his efforts against the town and go home.
Henry angrily replied that James had no right to summon him, and ought to be England's ally, as James was married to Henry's sister Margaret, declaring:For more information on this conflict, go here ..... Battle of Flodden