During Elizabeth’s reign, the biggest threat was from Spain, which was a powerful empire with a strong navy.
King Phillip II of Spain (who had been married to Elizabeth’s half-sister, Mary) wanted a Catholic ruler on the English throne. He was also very angry at Elizabeth, because she allowed her sailors to steal treasure from his ships.
In 1588, he sent 130 ships to invade England. This is known as the Spanish Armada.