John Steward, third earl of Buchan, was the second son of Robert Stewart, first duke of Albany (c.1340-1420), the de facto ruler of Scotland for most of the period from 1388 until his death. Buchan’s career was thus almost entirely dominated by his father’s political ambitions.
It was these ambitions that brought Stewart the title of earl of Buchan, reflecting his father's desire to build up a powerbase in the north east of Scotland. It also brought him a marriage to Elizabeth, the daughter of Archibald Douglas, fourth earl of Douglas, one of the most powerful Scottish nobles. In 1402 Buchan’s older brother Murdoch Stewart was captured at the battle of Homildon, and for the next few years Buchan acted in his stead as his father’s political heir, at least until Murdoch’s own sons came of age. Murdoch was finally exchanged for Henry Percy “Hotspur” in 1416.
This connection to the Douglas family brought Buchan an appointment as the senior commander of a 6,000 strong Scottish army that responded to a French call for aid in 1418-19. The majority of these 6,000 men were members of the Douglas retinue, and the command was shared with Archibald Douglas, earl of Wigtown, his brother-in-law. The first year of the Scottish expedition to France was quiet, with many of the men distributed in garrisons. 1420 also saw the death of Buchan’s father Robert Stewart. Buchan’s brother Murdoch inherited his titles and most of his posts.
The relative inactivity changed in 1421. Buchan commanded the Franco-Scottish army that won a victory at Baugé on 22 March 1421 which resulted in the death of Thomas, duke of Clarence, Henry V’s brother. Buchan was rewarded with the office of constable of France, making him the effective commander-in-chief of the French army. He was also given land at Châtillon-sur-Indre, and the services of an astrologer.
This was the high point of Buchan’s military career. Later in 1421 he took part in an unsuccessful siege of Alencon, which nearly led to another battle, and also in the successful attack on Montmirail in May-June. Early in 1423 he led an unsuccessful campaign in the Ile-de-France, before returning to Scotland at the head of an embassy seeking further reinforcements. Despite a series of obstacles he was successful in this, and in March 1424 returned to France as part of another large Scottish army, this time under the command of Archibald Douglas, fourth earl of Douglas (and now duke of Touraine).
By the summer of 1424 the English were besieging Ivry, close to Le Mans. A large Franco-Scottish army under the command of Douglas, Buchan, the duke of Alencon and the viscounts of Narbonne and Aumâle moved to relieve the siege, but before they could arrive, the town surrendered. After a council of war it was decided to attack some of the English possession on the border of Normandy. In mid-August the combined army captured Verneuil. The English commander in France, the duke of Bedford, responded quickly, leading an army south to Verneuil. The senior French commanders would have preferred to retire, but it seems that the Scots convinced them to make a stand. On 17 August 1424 the Franco-Scottish army suffered a heavy defeat. Both Buchan and Douglas were killed during the battle, along with around 4,000 of the Scottish contingent.
The defeat at Verneuil seriously weakened the position of Charles VII. It also had a significant impact on Scottish politics. James I had finally returned from captivity in England. The deaths of Douglas and Buchan removed two of the most influential Scottish magnates, and in the following year James ordered the arrest and execution of Murdoch Stewart, second duke of Albany. One unexpected side effect of the battle of Verneuil was a revival in the power of the Scottish kings.