Edward's own rule started on 19 October 1330. Mortimer and Isabella had called a great council at Nottingham, at which they questioned the King over his actions. On the 19th, Edward and a small group of allies entered Nottingham castle, possibly through secret caves, and captured Mortimer, who was taken to London and executed. Edward's true reign should be dated from this dramatic coup. His first miltary venture was against the Scots. The death of Robert the Bruce, leaving his infant son David II as king of Scotland, reopened the question of the succession. The first action was taken a group of dispossessed noblemen, who had lost their Scottish lands. They invaded Scotland in 1332, and won the battle of Dupplin Moor, making Edward Balliol a valid claimant to the throne of Scotland. Edward III now put his resources into a Scotish war, moved the centre of government to York, and began the campaign that led to his victory at Halidon Hill (19 July 1333). Halidon Hill saw Edward using the same tactics of mixing archers and dismounted men-at-arms that won him his great victories in France. David II was forced to flee to France, where in the Spring of 1334 Philip VI of France took him under his protection. Balliol now took a shaky hold over the Scottish throne, weakened considerably by Edward III, who in June 1334 took control of much of lowland Scotland as the price of his support. Balliol was unable to maintain his rule for long, and the main result of the entire episode was to produce a new cause for dispute between England and France.
On 24 May 1337, the first steps of the Hundred Years War (1337-1453) were taken, when Philip VI of France confiscated Edward's duchy of Aquitaine, officially at least because Edward had harboured Robert of Artois, Philip's cousin and enemy. Since the treaty of Paris (1259), the kings of England had been forced to hold Aquitaine as a fief from the kings of France, causing problem after problem. Edward decided to break this bond, and it was this as much as anything that made this war different from the many earlier short Anglo-French campaigns. In addition, Edward had a claim to the French throne as the grandson of Philip IV and nephew of Charles IV. This claim had first risen in 1328, when there was not realisitic way in which it could be persued, but in 1340 Edward took the title of King of England and France, creating a joint monarchy that was not abandoned by the Kings of England until long after the end of the Hundred Years War.
The first ten years of the war produced very little action of note other than the naval victory of Sluys, and it was not until the victory at Crecy (1346) and the siege of Calais (1346-7) that Edward's military reputation was made. At the end of the first phase of the war in 1360 Edward had made sizable gains in France, and in return for the grant of full sovereignty in those areas, he had agreed to renounce his claim to the French throne. Neither side implemented the treaty in full, and in 1369 the war was resumed. This last phase of the war went badly for the English. Edward III was increasingly elderly, while his most capable son, Edward, the Black Prince, was increasing ill, and died before his father. By the time of his death on 21 June 1377, Edward had seen all of his conquests bar Calais lost to the French.
Although Edward had lost most of his conquests by his death, his reign saw two major developments of importance here. First was the tactic combination that saw the English armies repeatedly beat larger French forces. He and his generals combined the increasingly powerful longbowmen, with dismounted men-at-arms, to fight a serious of defensive battles, starting at Halidon Hill and continuing long after Edward's reign, including Agincourt, forty years after Edward's death. The French were unable to find a way to defeat these tactics in battle, and were forced to use a strategy of refusing battle, which in the end frustrated all English efforts. The second result of Edward's reign was that the reputation of English arms reached unprecedented heights. Under Edward II, English armies had seen very little but defeat, against the Scots at Bannockburn and against the French in Gascony. Even Edward I had not had a great reputation on the continent, where his campaigns had been of little note. In contrast, Edward III's victories came against the French knights, seen as the greatest of their time, and often against greater numbers. While the mounted knight met defeats elsewhere in the fourteenth century, these great battles struck at the heart of France.