Final answer:
Napoleon Bonaparte's expedition into Egypt led Sultan Selim III to ally with Great Britain, with the invasion resulting in the Ottoman declaration of war supported by Britain. Pedro Cabral was the first explorer to reach Brazil and claim it for Portugal.
Step-by-step explanation:
The expedition that forced Sultan Selim III to ally with Great Britain from 1798 to 1801 was led by Napoleon Bonaparte. During this period, known as the French campaign in Egypt and Syria, Napoleon invaded Egypt in an effort to undermine British access to India and to establish French influence in the region. The invasion began on July 1, 1798, and included successes such as the Battle of the Pyramids. However, the British navy under Admiral Horatio Nelson defeated the French forces at the Battle of the Nile, isolating Napoleon's troops and leading to an Ottoman declaration of war against France with British support.
In regards to the explorer who reached Brazil and claimed it for the throne of Portugal, it was Pedro Cabral who accomplished this feat, distinguishing himself from other explorers such as Christopher Columbus, Ferdinand Magellan, and Jacques Cartier.