The movie is largely based on <i>Henry V</i> and Part 1 and 2 of <i>Henry IV</i> by Shakespeare, which means that it takes a lot of liberties in how it interpreted historical events. In fact, many characters and events, while historical, did not occur or result in how they are portrayed. For one, Thomas was never heir to the throne, as far as we know, and he died long after Henry IV died. He also died fighting the Scots at the Battle of Baugé in France. Henry V was also very much interested in war with France early on, although he may have seen this as a way to unite his own kingdom. Falstaff is a made up character and Henry V, rather than being somewhat irresponsible in his youth, was actually very active in his father's affairs from an early age. Already during the reign of his father, Henry was leading many of the battles against rebellions that occurred and thus was an experienced warrior before he became king. Henry V did fall out for a while with his father, but soon much of the kingdom rallied around Henry V after his father died. Additionally, William Gascoigne resigned soon after Henry V became king. He, in fact, had little influence on the king and events surrounding the war in France.
The depiction of key events such as the Battle of Harfleur and Battle of Agincourt are accurate, as well as some of the events as to how the English gained these key victories. Mainly, the English longbow played a decisive role in the French campaigns, as the French did not have an effective counter to this. The English were able to kill many leading French nobles, creating a crisis in France that led to the French king offering his daughter to the English king. The Dauphin also was never present in the Battle of Agincourt. In fact, he died soon after from dysentery. In fact, the death of the Dauphin and Henry's success led to the French king wanting to make Henry the future king of France. This never happened because Henry V died from his own infection of dysentery and the crowns of England and France proved they could not be united so easily as the French reneged the treaty signed earlier that would have united the two kingdoms. Overall, <i>The King</i> does appear a lot like the plays by Shakespeare, which were effectively propaganda for Henry V. The events concentrate on the events leading to the Battle of Agincourt, which showed the king as a mature war leader. However, in reality Henry V was groomed as heir apparent and as a war leader early on in his life and he was already successful militarily before he became king.