The Battle of Manila Bay, also known as the Battle of Cavite, took place on May 1, 1898, during the Spanish–American War. The American Asiatic Squadron under Commodore George Dewey engaged and destroyed the Spanish Pacific Squadron under Contraalmirante Patricio Montojo. The battle took place in Manila Bay in the Philippines, and was the first major engagement of the Spanish–American War. The battle was one of the most decisive naval battles in history and marked the end of the Spanish colonial period in Philippine history.

Tensions between Spain and the United States worsened over the Spanish conduct during their efforts to quell the Cuban War of Independence, with many Americans being agitated by largely falsified reports of Spanish atrocities against the Cuban population. In January 1898, fearing the fate of American interests in Cuba due to the war, the cruiser USS Maine was dispatched to protect them. Less than a month later, the battleship exploded while lying at anchor in Havana harbor, killing 261 sailors onboard and inflaming American opinion.

Upon the outbreak of war, the Americans realized that defeating a significant Spanish squadron then stationed in the Philippines was important to ensuring victory in the war. The U.S. Asiatic Squadron commanded by Dewey, a veteran of the American Civil War, was dispatched to ensure success. On 1 May, the American squadron steamed into Manila Bay to engage with the Spanish. The Spanish, aware that they were hopelessly outgunned, made a desperate defense against the Americans. The battle was not much of a contest, with superior American naval gunnery and seamanship ensuring the entire Spanish fleet would be sunk with minimal casualties for the Americans, who suffered only ten casualties in all. Upon realising that the battle was hopeless, Montojo ordered his two protected cruisers to be scuttled to ensure that they did not fall into the hands of the Americans. The battle remains one of the most significant naval battles in American maritime history.

Americans living on the West Coast of the United States feared a Spanish attack at the outbreak of the Spanish–American War. Only a few U.S. Navy warships, led by the cruiser USS Olympia, stood between them and a powerful Spanish fleet. In practice, however, Olympia was far superior to the Spanish colonial fleet, as the battle would show.

Admiral Montojo, a career Spanish naval officer who had been dispatched rapidly to the Philippines, was equipped with a variety of obsolete vessels. Efforts to strengthen his position amounted to little. The strategy adopted by the Spanish bureaucracy suggested they could not win a war and saw resistance as little more than a face-saving exercise. Administrative actions worked against the effort, sending explosives meant for naval mines to civilian construction companies while the Spanish fleet in Manila was seriously undermanned by inexperienced sailors who had not received any training for over a year. Reinforcements promised from Madrid resulted in only two poorly-armored scout cruisers being sent while at the same time the authorities transferred a squadron from the Manila fleet under Admiral Pascual Cervera to reinforce the Caribbean. Admiral Montojo had originally wanted to confront the Americans at Subic Bay, northwest of Manila Bay, but abandoned that idea when he learned the planned mines and coastal defenses were lacking and the cruiser Castilla started to leak. Montojo compounded his difficulties by placing his ships outside the range of Spanish coastal artillery (which might have evened the odds) and choosing a relatively shallow anchorage. His intent seems to have been to spare Manila from bombardment and to allow any survivors of his fleet to swim to safety. The harbor was protected by six shore batteries and three forts whose fire during the battle proved to be ineffective. Only Fort San Antonio Abad had guns with enough range to reach the American fleet, but Dewey never came within their range during the battle.

The Spanish squadron consisted of seven ships: the cruisers Reina Cristina (flagship), Castilla, Don Juan de Austria, Don Antonio de Ulloa, Isla de Luzon, Isla de Cuba, and the gunboat Marques del Duero. The Spanish ships were of inferior quality to the American ships; the Castilla was unpowered and had to be towed by the transport ship Manila. On 25 April, the squadron left Manila Bay for the port of Subic, intending to mount a defense there. The squadron was relying on a shore battery which was to be installed on Isla Grande. On 28 April, before that installation could be completed, a cablegram from the Spanish Consul in Hong Kong arrived with the information that the American squadron had left Hong Kong bound for Subic for the purpose of destroying the Spanish squadron and intending to proceed from there to Manila. The Spanish Council of Commanders, with the exception of the Commander of Subic, felt that no defense of Subic was possible with the state of things, and that the squadron should transfer back to Manila, positioning in shallow water so that the ships could be run aground to save the lives of the crews as a final resort. The squadron departed Subic at 10:30 a.m. on 29 April. Manila, towing Castilla, was last to arrive in Manila Bay, at midnight.

At 7:00 p.m. on 30 April, Montojo was informed that Dewey's ships had been seen in Subic Bay that afternoon. As Manila Bay was considered unnavigable at night by foreigners, Montojo expected an attack the following morning. However, Oscar F. Williams, the United States Consul in Manila, had provided Dewey with detailed information on the state of the Spanish defenses and the lack of preparedness of the Spanish fleet. Based in part upon this intelligence, Dewey—embarked aboard Olympia—led his squadron into Manila Bay at midnight on 30 April.

Passing the entrance, two Spanish mines exploded but were ineffective as they were well below the draft of any of the ships due to the depth of the water. Inside the bay, ships normally used the north channel between Corregidor Island and the northern coast, and this was the only channel mined. Dewey instead used the unmined south channel between El Fraile and Caballo Islands. The El Fraile battery fired a few rounds but the range was too great. The McCulloch, Nanshan and Zafiro were then detached from the line and took no further part in the fighting. At 5:15 a.m. on 1 May, the squadron was off Manila and the Cavite battery fired ranging shots. The shore batteries and Spanish fleet then opened fire but all the shells fell short as the fleet was still out of range. At 5:41 with the now famous phrase, "You may fire when ready, Gridley", the Olympia's captain was instructed to begin the destruction of the Spanish flotilla.