Final answer:
Spanish leaders in Mexico were concerned about Philip Nolan because they suspected him of being an American spy during a time of considerable territorial competition in North America.
Step-by-step explanation:
Philip Nolan concerned Spanish leaders in Mexico because he was suspected of being an American spy. Nolan's activities in Spanish territory sparked alarm among Spanish officials who were wary of American expansionist tendencies. The early 19th century was a period of intense geopolitical maneuvering, with various European powers and the newly independent United States vying for influence and territory in North America.