Henry IV Part 1: Theme Analysis

The Development of Prince Hal
At the beginning of the play, Henry IV laments the fact that his son, Prince Hal, leads a wild life. The Prince forgets his responsibilities as heir to the throne and consorts with low company in taverns. A number of early scenes show Prince Hal behaving in exactly the way the King deplores-hanging around with Falstaff and his companions who are planning a robbery, for example.
But during the course of the play, Prince Hal reforms his behavior and shows himself to be a worthy Prince of Wales and heir to the throne. He announces as early as Act 1, scene 2 that he is only going along with his disreputable companions for a while. He fully intends to shake them off when the time comes for royal duty. At key points in the play, Hal's evolution is shown. He is a changed man after his private interview with the King. His performance at the battle of Shrewsbury, in which he saves the King from Douglas and also kills the rebel leader Hotspur, shows he has what it takes to be the future Henry V. He demonstrates a steadiness of temperament that the impulsive Hotspur lacks. In addition to his prowess in battle, Prince Hal is also gracious and magnanimous in victory, as is seen by his willingness to release Douglas without a ransom. He is a different man by the end of the play than the one the audience saw at the beginning.
Honor
The theme of honor is first introduced by Hotspur. Honor is what motivates him. He believes he is upholding honor by leading a rebellion against an unjust king. This is shown by his speech in Act 1, scene 3, lines 199-206, in which he declares that "drowned honor" must be plucked up from the bottom of the deep. In other words, he feels that honor has been disgraced by the king's rule and he want to resurrect it.
Although on the royal side there is no direct mention of honor, the concept nonetheless underlies the entire conflict. The King's urgings to his son convey the idea that it is time for the young man to behave honorably and help to defeat the rebels. The need to uphold honor is what leads the men on both sides into battle. No one can back down and still retain honor; they must uphold the martial code. Falstaff, however, satirizes the concept of honor. For him, honor is just a meaningless word. It has no practical use, and is certainly not worth sacrificing one's life for (see his speech Act 5 scene 1).
Legitimacy of Kingship
During this turbulent period in English history, there were heated disputes about who was the legitimate ruler of the country. This theme underlies the play. Since Richard II, the undisputed legitimate king, had been overthrown, both sides in the civil war must argue the case for their own legitimacy. This is why the King feels it necessary, when speaking in private to his own son (Act 3, scene 2), to justify his own actions in seizing the crown from Richard II. He upholds his right to rebel against a legitimate ruler who was behaving unjustly. However, in this scene he also wonders whether God is punishing him, through his irresponsible son, for some sins on his part. This suggests a guilty conscience, or at least some uncertainty about his own actions.
The rebels do not believe Henry IV is a legitimate ruler, and they justify their rebellion on this basis, as well as spelling out their specific grievances. Of course, had the rebels succeeded, it is open to question whether their rule would have had any more legitimacy than that of Henry IV.
Shakespeare seems clearly to side with the royal cause. He shows that both Henry IV and his son have the necessary personal qualities of courage and chivalry that justify their rulership over the country, however dubious may Henry IV's overthrow of Richard II been.