I believe that a major theme of Henry V is leadership and how it relates to morality. As King, Henry has to make decisions that would be immoral if he were just a common man. He betrays his friends and puts people to death to uphold the law. But since he is King, he is seen as brave, intelligent and incredibly focused. He is a great ruler and is able to inspire many men.
I think another theme is related to the problems within the country and how they may be forgotten when war is waged against France. When the attention of the people is focused elsewhere they will forget about the issues at home and Henry knows this.