What is the theme in King Henry V by William Shakespeare?

Patriotism is definitely a theme in this play. Consider the many rousing speeches that Henry gives his underdog troops before charging into battle.

But we in it shall be remember'd; We few, we happy few, we band of brothers; For he to-day that sheds his blood with me Shall be my brother...

Henry makes his soldiers believe that God is on their side and that he, a king, will be their brother on the battlefield.