By 1314, Bruce had recaptured most of the castles in Scotland held by the English and was sending raiding parties into northern England as far as Carlisle. In response, Edward II planned a major military campaign with the support of Lancaster and the barons, mustering a large army of between 15,000 and 20,000 men. In the spring of 1314, Edward Bruce laid siege to Stirling Castle, a key fortification in Scotland whose governor, Philip de Mowbray, agreed to surrender if not relieved before 24 June 1314. In March, James Douglas captured Roxburgh, and Randolph captured Edinburgh Castle (Bruce later ordered the execution of Piers de Lombard, governor of the castle), while in May, Bruce again raided England and subdued the Isle of Man. News of the agreement regarding Stirling Castle reached the English king in late May, and he decided to speed his march north from Berwick to relieve the castle. Robert, with between 5,500 and 6,500 troops, predominantly spearmen, prepared to prevent Edward's forces from reaching Stirling.

The battle began on 23 June as the English army attempted to force its way across the high ground of the Bannock Burn, which was surrounded by marshland. Skirmishing between the two sides broke out, resulting in the death of Sir Henry de Bohun, whom Robert killed in personal combat. Edward continued his advance the following day, and encountered the bulk of the Scottish army as they emerged from the woods of New Park. The English appear not to have expected the Scots to give battle here, and as a result had kept their forces in marching, rather than battle, order, with the archers − who would usually have been used to break up enemy spear formations − at the back, rather than the front, of the army. The English cavalry found it hard to operate in the cramped terrain and were crushed by Robert's spearmen. The English army was overwhelmed and its leaders were unable to regain control.

Edward II was dragged from the battlefield, hotly pursued by the Scottish forces, and only just escaped the heavy fighting. In the aftermath of the defeat, Edward retreated to Dunbar, then travelled by ship to Berwick, and then back to York; in his absence, Stirling Castle quickly fell.