When Henry hears at Agincourt that the French have sent in reinforcements, he orders his men to kill their French prisoners--a terrifying act for men facing the prospect of imprisonment themselves.
Henry sent his men to kill the Spanish soldiers. 
mnli hypothesis: Henry sent his men to kill the Spanish soldiers.  premise: When Henry hears at Agincourt that the French have sent in reinforcements, he orders his men to kill their French prisoners--a terrifying act for men facing the prospect of imprisonment themselves.