In Shakespeare's play Henry IV, illusion vs. reality is a common theme. For instance, the king admits he wishes that his son Hal were the gallant Hotspur rather than the lazy rebel he truly is. This "illusion" Henry has for his son creates friction between the two and make the royal family a less than united front.
In addition, Hotspur and the rebels are under the impression that they can easily defeat the king and his supporters. However, this cocky attitude only leads to their ultimate demise, as they were not sufficiently prepared for the real battle- only the simple ones they had created in their minds.
Blunt also dresses up as the King so the rebels will go after him rather than the king. Douglas falls to his trap, and because of the disguise believes he kills Henry, when in reality he is very much alive. This demonstrates how things are much larger and more complex than they may appear, and that we should not blindly trust one action solve problems; rather, it takes persistence and knowledge.