According to Holinshed's Chronicles (a primary source for Shakespeare's history plays):
In Lent [in the year 1414], while the King was at Kenilworth, certain Ambassadors came to him from Charles, Dauphin of France, the eldest son of the French King. They brought with them a barrel of Paris balls, which they presented to him as a token from their master. This present was taken in very ill part, as sent in scorn, to signify that it was more meet for the King to pass the time with such childish exercise, than to attempt any worthy exploit. Wherefore the King wrote to him, that before very long he would send him some London balls, that should break and batter down the roofs of his houses about his ears.
[I've edited this into something more like modern English.]
Modern scholars believe that the gift was probably sent in good part, as tennis was very fashionable at the time. They also doubt whether the Dauphin would have deliberately offended his country's most dangerous enemy. Some have speculated that Henry and his advisers may have deliberately orchestrated the 'gift', to provide an excuse for going to war.
Whatever the truth of the matter, Henry sailed for France in August 1415 with 10,500 fighting men. But by late October he found himself outmanoeuvred, low on supplies, and forced to make a stand against a much larger French army at Agincourt.
The rest is history ...
© Haydn Thompson 2017