Final answer:
Assistant Secretary of the Navy Theodore Roosevelt sent a telegraph to Commodore George Dewey, instructing him to prepare the Pacific fleet for an attack on the Spanish fleet in the Philippines, ultimately resulting in a decisive US victory at the Battle of Manila Bay.
Step-by-step explanation:
Months before the Spanish-American War was declared, Assistant Secretary of the Navy Theodore Roosevelt sent a telegraph to Commodore George Dewey, instructing the Pacific fleet to move to Hong Kong and prepare for operations against the Spanish fleet in the Philippines in anticipation of the conflict. This preemptive move was made possible when Roosevelt's superior, Secretary of the Navy John D. Long, was out of the office, allowing Roosevelt to issue commands for the fleet's deployment.
The resulting telegraph to Commodore Dewey initiated a chain of events leading to the seminal Battle of Manila Bay on May 1, 1898, where the American fleet decisively defeated the Spanish forces, with negligible American losses. Roosevelt's actions were influenced by Alfred T. Mahan's strategic recommendations on asserting naval power and constructing a modern fleet, as well as by broader American imperialist and industrialist interests seeking new markets, particularly in China.