Note to Hamlet, 4.4.53-56: "Rightly to be great / Is not to stir without great argument, / But greatly to find quarrel in a straw / When honor's at the stake"
I think Hamlet is being extremely sarcastic. The traditional idea about greatness in warfare is that the leader should "not stir without great argument"; that is, if a leader is going to risk the lives of his men he should do so only with a "great argument," a just cause, such as homeland defense. Hamlet sarcastically turns this whole idea on its head and says that now all a leader (such as young Fortinbras) needs to be "great" is to "find quarrel in a straw." Fortinbras, whose father died in single combat with Hamlet's father, apparently thinks his honor is at stake, and so is determined to win the honor of beating the Poles out of a "little patch of ground" not worth five ducats.