The English Prince got careless, and with a rather small number of men, got trapped by a far larger force of the French king.
Here is Froissart's version of his address to them when the battle is about to start:
"Therefore, sirs, for God’s sake I require you do your devoirs this day; for if God be pleased and Saint George, this day ye shall see me a good knight."

I thought "devoir" was a French word, and was actually surprised to find it in my dictionary, defined as "duty".

devoir
n.
5ME < OFr, to owe < L debere, to owe: see DEBT6
1 duty
2 [pl.] acts or expressions of due respect or courtesy, as in greeting