English privateers played a significant role in challenging the Spanish Empire primarily by stealing the contents of ships. These privateers were essentially state-sanctioned pirates who were authorized to attack and plunder enemy vessels and their cargo. This was especially prevalent during the late 16th and early 17th centuries, during periods of intense rivalry between England and Spain, such as the Anglo-Spanish War.
Privateers targeted not only Spanish treasure ships returning from the Americas but also Spanish settlements and shipping routes. While they did not directly seize territory in the Americas as a primary objective, their activities weakened Spanish maritime power and disrupted trade, which helped to establish Anglo-American dominance in certain areas over time.
In summary, the most accurate role of English privateers in challenging the Spanish Empire is that they stole the contents of ships, undermining Spanish economic strength and contributing to the shift of power away from Spain.