Henry focused his intentions single-mindedly on regaining the lands in France and galvanized Parliament into total support. His initial negotiations with the French proved fruitless - not surprisingly considering Henry's wish to have the whole of the old Angevin Empire restored. When negotiations ceased, Henry invaded on II August 1415. He successfully besieged Harfleur and then, on 25 October, confronted the French army at Agincourt. Although the French outnumbered the English three-to-one, their rash tactics against the English and Welsh longbow men proved disastrous. The flower of French chivalry died on that day. It is reported that the French lost some six thousand men compared to four hundred English.

Although Agincourt was a major victory, it was not decisive because France did not immediately fall to Henry. But his success had caused the French to be extremely wary of Henry, and it earned Henry tremendous support in England. He also succeeded in gaining Sigismund, the Holy Roman Emperor, as his ally and together they won the support of John, duke of Burgundy. Starting again in August 1417, Henry pursued his campaign through Normandy, which fell to him in the spring of 1419. A few months later the duke of Burgundy was murdered, and Henry found that his own claim on the French throne was now supported by all Burgundians in total defiance of the Armagnac party, adherents of the Dauphin, Charles. Henry signed the Treaty of Troyes with the Burgundians in May 1420, which recognized him as the heir to the French throne. To seal the alliance Henry married Katherine, daughter of the French king. Henry brought his bride back to England in February 1421 and spent the next few months in a triumphant tour of the land. It was soured slightly by the news of the death of his brother, Thomas, duke of Clarence, at the battle of Bauge in France in March. By June Henry was back in France, continuing his battle for the kingdom. The Dauphin's stronghold at Meaux fell to him in May 1422 after a long siege. Unfortunately Henry had become very ill during the winter, almost certainly with dysentery, and his health worsened during the summer. He died on 31 August 1422, aged only thirty-four. It was a cruet blow of fate. Less than six weeks later the French king died. Like Moses, Henry was in sight of his kingdom but was denied entry into it. Nevertheless Henry bequeathed to his infant son a kingdom greater than any since the days of Henry II.

Our image of Henry is probably drawn mostly from his portrayal by Laurence Olivier in the film version of Shakespeare's Henry V, but Olivier's own portrayal was based on a painting of Henry that shows a rather humourless, determined young man with a hard face, an over-large nose, thin eyebrows and a clipped monkish haircut. Although Henry's reign had been the shortest of any English king since the Norman conquest (even Richard I's was three months longer), it was one of the most successful. No other king gained such united support from his barons and no other king regained so much territory and held it. There is no way of knowing how Henry would have maintained his two kingdoms had he lived and his reign might have ended in failure, like Edward III's. But Henry was also a tireless administrator who, despite the pressure of his military campaigns, continued to respond to petitions from England and meet the needs and demands of his court. His ability to hold together his parliament despite the immense strain upon the treasury of his campaigns is further testimony to his abilities. Henry also remained fond of and on good terms with his step-mother, Joan of Navarre, even though she was accused of witchcraft in 1417 and imprisoned at Pevensey Castle. His ability to handle all of these eventualities and triumph in them all was remarkable. His early death was a tragic loss to England. His wife, Katherine, subsequently formed a romantic alliance with Owain Tudor, and most probably married in secret. By him she had four or five children, and died giving birth to the last in 1437. By her son Edmund, she was the grandmother of Henry VII.