Even before war broke out Assistant Naval secretary Teddy Roosevelt had sent Admiral Dewey commander of US naval forces in the western to be prepared if war came with Spain to proceed to Manila and defeat the Spanish fleet there.
When war was declared Dewey received orders to proceed and destroy the Spanish fleet. Admiral Dewey commanded a squadron of six ships, including modern cruisers.
The Spanish had a force of 10 old ships, which were no match for the American ships.
The Spanish commander Admiral Patricio Montojo decided to fight the battle anchored in Manila Bay hoping, to gain some advantage from shore batteries. Dewey acted aggressively, entering Manila Bay at night, thus providing the Spanish very little time to prepare for battle.
Leading the American force was the Olympia with Dewey aboard. At 5:15 in the morning of the first of May, the Spanish battery at Cavite opened fire on the American force. The American fleet continued and at 5:41 at a distance of 4000 yards Dewey gave the order to open fire.
The American squadron steamed up and down the Spanish line five times, laying devastating fire on the Spanish ships. At 7:35 Dewey temporarily withdrew believing mistakenly that his ships had only 15% of their ammunition left.
After it was ascertained that only 15% had been used, Dewey resumed his attacks at 11 against a rapidly sinking Spanish force. At 12:30 the battle ended, with the entire Spanish fleet sunk.
cr: History Central (Link)