The play celebrates the victory of the English over the French at the Battle of Agincourt during the Hundred Years' War. At the time, the victory was considered an amazing upset because the English were greatly outnumbered by the French. The play gives most of the credit to Henry V for the victory. However, historians today say that Henry benefited greatly from the invention of the longbow which allowed his men to stand at a distance and kill the French troops. At the time, the French did not have the longbow and this gave Henry a great tactical advantage. Shakespeare wrote his play 200 years after the actual events and, included the St. Crispen's Day speech, which is probably fictional, but is known to this day as one of the best speeches of Shakespeare.
St Crispians day :)