The film features a lot of historical deviations regarding The Hundred Years War, some taken from William Shakespeare himself and some being entirely new.
- Hal and Hotspur did not engage in single combat at Shrewsbury. There was in fact a big battle there, where the Percy rebels were defeated and Hotspur was killed in combat (in the play the Percys refuse Hal's single combat offer, but Hal and Hotspur still meet to fight in the middle of the battle).
- As in Shakespeare, the movie has Hotspur and Henry being roughly the same age, when in real life Hotspur was over 20 years older than Prince Hal. He also died about 10 years before Henry succeeded.
- The Dauphin was not at Agincourt. He died of natural causes, probably dysentery, a few months later.
- Hal's drunken youth, which Shakespeare took from folklore, probably never happened (he was fighting in his father's battles as a teenager). There is also no firm evidence for the old story, also in Shakespeare, about the Dauphin taunting Henry with a gift of tennis balls.
- William Gascoigne (a character who does not appear by name in Shakespeare's play) did not die until 1419, a full four years after Agincourt, which resulted from natural causes, not Henry murdering him. There's no evidence for him ever fabricating a plot to get Henry into the war either.
- While Henry V and Timothee Chalamet share a slender build, Henry was also exceptionally tall for his era at 6'3", while Timothee Chalamet's height of 5'10" is fairly average for modern times.
- Henry's younger brother Thomas died six years after Agincourt. Though he chose his father's side in the conflict between Henry IV and Prince Henry, there were no plans to put Thomas on the throne instead.
- Henry is portrayed as a reluctant warrior who prefers peace. Actually, he pushed for retaking English-held lands in France prior to his father dying, and started the war for this of his own accord after becoming king. The real man also had a much stronger personality than he's portrayed with in the film. His lords actually counseled Henry against war before he had negotiated extensively, much the opposite from what the film portrays.
- The siege of Harfleur in real life took place very differently. First of all, it was a port city, not just a castle (and even more important to capture as a bridgehead of the English invasion). The garrison didn't surrender until after the walls were breached, which was done by cannons rather than trebuchets. Henry also lost a quarter to a third of his men from dysentery during the siege (while the threat of disease is mentioned in the film, this isn't shown at all).
- The battle of Agincourt also took place very differently in real life. Specific examples include:
- The role of English/Welsh archers being severely downplayed. In the real battle, the French were advancing under a consistent Rain of Arrows and this played a crucial part in the decisive English victory. Only a couple volleys are shown and the archers themselves are positioned incorrectly, set up directly behind the English front line instead of at the flanks, hidden by the trees.
- While the film is one of the few to accurately portray the essential role of the heavily armored line of dismounted knights at the English center, the way the battle is depicted makes Falstaff's plan of baiting the French into a suicidal countercharge against it downright stupid because the flanks of this force are exposed. The French knights could just envelope them. In the actual battle, the English center was anchored by forest at the flanks. A small force of French knights did manage to slip past the English and attack their baggage train, but were unable to coordinate with the rest.
- Henry commanded the center of the English dismounted knights, whereas in the film he commands a flanking force.
- The French knights would not have been able to charge the English lines at Foe-Tossing Charge speed because their horses were frightened or being killed by the arrow barrage, and the muddy ground slowed their advance to an agonizing crawl. Oddly, the muddy ground is mentioned, but plays no part in how the battle is filmed. The English knights are quickly overwhelmed and several French knights are shown charging right through with contemptuous ease. In the real battle, this alone would have probably won the battle for France.
- Few French noblemen are shown being captured or killed. The battle was infamous for having a casualty list which included virtually every leader of northern France at the time. In addition, the Dauphin is killed off when he wasn't even present (both in the play Henry V and in history), and casually ordering his death instead of holding him for an astronomical ransom would have been unthinkable.
- The film also indicates that King Charles made peace immediately after Agincourt, while Henry then married Catherine as a result. In fact, this didn't take place until five years later, with many battles being fought before that actually happened.
- The armor worn is not strictly accurate, nor the drab colors of clothes worn (at least by the nobles, who liked bright hues). Additionally, at the time in the royal palace there were bright, lavish decorations on the walls.
- Princess Catherine utters the very anachronistic view that all monarchy is illegitimate. Though some might have felt that then, it's highly improbably she did, especially being a princess herself. She also would likely not have spoken to Henry in such a bold way (even slightly disparaging) at least when they had just met and Catherine didn't know what he'd tolerate. Nor would arranged marriage have been odd to her, as this was actually the norm for royalty at the time.
- When he shows up at Shrewsbury, Henry assures Thomas that he hasn’t come to “steal [his] thunder”, a phrase not coined until the 18th century—ironically enough, it relates to people filching techniques of generating sound effects in theatre plays.
