Inspirational speeches can be either extremely helpful or extremely annoying. Henry V's motivational speech during Act IV Scene iii-v was meant to motivate his soldiers and see the bright side of the seemingly impossible battle before them. He knew that his army was hugely outnumbered compared to the French's but he figured he could at least give his troops some type of hope. False hope maybe, but hope nonetheless.
There are many themes that could come out of this inspirational speech. There was warfare, patriotism, glory, honor, power and many more. The one that was most prominent in the speech was honor. Henry told his soldiers that even if they were to loose, they would earn immense honor for their generations to come, simply because they fought along their king in tough times. That meant the world to these soldiers because a mans reputation, especially if it involved their family, was more important to them than loosing their lives for an ill fought battle. As long as their family was safe from social suicide they would be happy. That's just how their society was run.
Shakespeare's purpose for writing this speech could've been for many reasons. St. Crispin's speech is still popular even hundreds of years later. For that time period, it was most likely patriotism. He wanted to show that England wasn't as weak as France thought. They were a type of "underdog" when it came to this war. Even Shakespeare himself seemed to question why Henry would even declare a war on France.
Whenever anything involves war, it's doomed to be gothic. Henry V is a war tale and at times may seem patriotic but mainly shifts between pride and horror. Shakespeare had pride that England was able to over come that oppression they felt they had with wining battles they seemed destined to loose. He also showed how war can be horrific, the effect on soldiers, their families, even how royals were affected. So Shakespeare's tone shifted throughout the whole story but the tone of the speech was for the soldiers benefit. They needed a boast of morale for the battle and that was supplied graciously by Henry.
In conclusion, the speech is a beautiful poem. Written in old English, difficult to understand but fun to analyze. No one can fully understand anything Shakespeare has ever written but we know it has a significant meaning that will benefit anyone timelessly.