How did the battle of Agencourt change the course of the One-Hundred Years War?

Agincourt had an opposite effect on the outcome of the war than a victory usually does, and it certainly prolonged it. Henry V's smashing victory over the French did nothing to prevent the loss of almost all English territory in France during the reign of Henry VI. Instead of making Henry VI king of both England and France it pretty much created French nationalism and set the stage for Joan of Arc.

In terms of casualties it was a terribly lopsided battle. The French were stupid and their chevaliers and coutilliers charged at the English defensive positions and were cut to bits by the English longbowmen. Some of them got past the smaller English line and chopped up the baggage train and camp. Nobody is really sure how many died on either side, though the English claimed only 115 of their own died. 500 is more likely, but perhaps as many as 4,000 French died, though naturally some claims are as high as 15,000.

It also had an effect on the art of war that lasted all the way through WWI. Defensive field formations became preferred by pretty much every general from 1415 through 1917. Everything from the pike hedgehogs of the 16th century to the infantry squares of the Napoleonic era were influenced by Crecy and Agincourt.

It was a moment of glory for Henry V and the genesis of one of the greatest speeches of Shakespeare, but it really did the English little good.

Agincourt was a victory for Henry V that was unexpected. He had his army on it's way back to Calais to return to England for the winter, when a French force caught up with him. A very bad spell of weather made the ground particularly muddy.

When the battle was joined the following morning, the flower of French knighthood was destroyed on the field, by the longbow. One particular quality the longbow had was not just it's range, and the greater rate of fire it had over the crossbow (it takes less time to notch and draw an arrow on a longbow than to put the crossbow down, hold it down with your foot, and try to pull the string back to the lock before you load the dart into it's spot), was the fact that arrows are not usually held in a quiver in battle, they're stuck into the ground in front of the archer. The ground has been used for farmland, and it's also had horses and cattle roaming over it. This means that it has been fertilized, one way or another, and getting struck, or even grazed with an arrow, can cause an infection that led to death.

The French forces were almost wiped out, with very few losses among the English soldiery. However, the luggage didn't manage as well. The Baggage train, with the young boys, women, old men, horses, food, and other supplies was attacked and most of them slain.

We few, we happy few, we band of brothers;

For he to-day that sheds his blood with me

Shall be my brother; be he ne'er so vile,

This day shall gentle his condition:

And gentlemen in England now a-bed

Shall think themselves accursed they were not here,

And hold their manhoods cheap whiles any speaks

That fought with us upon Saint Crispin's day.

Harry from Henry V by William Shakespeare Act IV Scene 3

two good answers above.

Here's a thought to develop........it is not that far in space or time from the longbow of the sturdy yeoman farmers of England at Agencourt to the the muskets of the New England farmer Minutemen at Lexington and Concord