ngland's attempt to locate a northwest passage and the subsequent discovery of "gold" in the Arctic attracted the attention of rival European nations. Spain in particular was interested, initially because it feared the expeditions might infringe on Spanish territory, and later because it did not want England to compete with Spanish control of a large portion of the world trade in precious metals.
In 1578, King Philip II of Spain sent Don Bernardino de Mendoza to London to replace the Spanish ambassador, who had been imprisoned for his role in a plot to drive the English out of Ireland. Understandably, the English were suspicious of Spain's new representative. On occasion his coded correspondence with King Philip was seized and decoded. However, other letters did arrive in Spain.
Through recent examination and deciphering of these letters, it has become clear that Mendoza had an informant on Frobisher's 1578 voyage. In a letter to the king dated October 7, 1578, Mendoza reported that Frobisher's ships had returned to England, but that he had not heard from his informer on the voyage. Yet his November 15 letter provides a relatively accurate summary of Frobisher's activities on that expedition. A chart of the voyage and ore samples were smuggled to him.