During the McKinley administration, John D. Long served as Secretary of the Navy. Long was a cautious and prudent official, far unlike his brash underling, Assistant Secretary of the Navy (and future US President) Theodore Roosevelt.
Roosevelt, sometimes called Teddy Roosevelt or TR, born sickly and nearsighted, had worked hard to improve himself into a physically powerful man through constant exercise. An active hunter, rancher, and nature-lover, Roosevelt loved competition and challenges. Not surprisingly, he was a fierce opponent of the long tradition of American isolationism, the ideology that suggested that the US should mind its own business and stay out of world affairs. Roosevelt thought the US should take a bigger role in shaping world affairs. In the case of the Spanish-American war, he was one of the most extreme "hawks" (pro-war), constantly pushing for war and always criticizing McKinley for seeming to be a "dove" who was afraid to go to war. McKinley, haunted by memories of the Civil War just three decades earlier, did not take the prospect of going to war as lightly as Roosevelt.
Along with being a lover of nature and competition, Roosevelt, despite the rugged image he projected, also was extremely well read. Based partially on his understanding of the leading military theorists of the time, and partially on his desire to see a wider conflict against Spain, Roosevelt wanted more than just a war in Cuba. For that reason, one weekend while his boss John D. Long was away, Roosevelt used his authority as Assistant Secretary of the Navy to cable Commodore George Dewey, the officer in control of the US Asiatic Squadron then docked at Hong Kong harbor, with orders that if the US and Spain went to go to war, Dewey was to immediately attack the Spanish fleet at Manila, the capital of the Philippines. Roosevelt gave these orders without Long's knowledge (and probably against Long's will) on February 25, 1898. Dewey figured something was fishy about these strange orders, and so double-checked the orders with McKinley. Strangely, McKinley, who had previously been agonizing over whether to go with war with Spain over Cuba, approved the surprise attack against the Spanish in the Philippines.
On May 1, 1898, Dewey's squadron, consisting of six brand new warships, sailed into Manila harbor. The 10-ship Spanish fleet was completely taken by surprise. Several of the Spanish ships were so old and rotting that they could barely float. Dewey's forces quickly defeated the Spanish fleet, without a single US sailor dying. On the Spanish side, around 400 sailors died. The Maine, which most Americans than believed had been destroyed by a Spanish mine, was avenged.
At the turn of the nineteenth century, some military experts in Europe believed the US was in for a difficult war with Spain. Their reasoning seemed sound: the US had no experience fighting in a tropical environment while Spain had a good deal of such experience. Also, Spain was an old nation that had fought many international wars while the US had spent most of the 19th century isolated from world affairs. Furthermore, in this island war, the relative powers of the two countries' Navies would be critical. Spain's Navy had been famous, if unlucky, all the way back to the days of the great Spanish Armada. In terms of the number of ships and the fighting experience of their captains, Spain looked to have an impressive Navy the US would have great difficulty matching.
However, Spain's Navy was really not as powerful as the other European nations believed. While the Spanish did have large numbers of ships, these vessels were old, rotting, and falling apart. The Spanish ships were no match for the newer ships of the US Navy, especially the American Navy's steel warships. In fact, the European experts were wrong: it was the Spanish Navy that was no match for the US fleet.
In general, the US and the European countries were focusing more and more on naval power in the 19th century, thanks to a groundbreaking book by US Captain Alfred Thayer Mahan called The Influence of Sea Power Upon History. In this book, widely read on both sides of the Atlantic, Mahan convincingly argued that the most powerful nations of the previous eras had always been the ones with the most powerful navies, from the Athenians on. Britain, which had controlled the seaways for much of the 19th century, was a prime example. Now, after reading Mahan's books, European and American leaders sought to build strong navies to protect their countries' interests and trade around the world. The US and other countries, especially Germany, started building world-class navies during this time thanks largely to Mahan's influence.
Why was Roosevelt so eager to have Dewey attack the Spanish fleet in the Philippines, a move that certainly would not help in the liberation of Cuba? Furthermore, why would the cautious McKinley ever approve such a move? The reason involves Mahan's theories yet again. In order to protect trade and influence throughout the world, Mahan advocated a series of island coaling stations throughout the world. (Since US ships ran on coal at the time, they needed places to stop and refuel) Roosevelt and McKinley hoped taking the Philippines from the Spanish would provide the US with a coaling station to help the US Navy patrol in the Far East, keeping Asian markets open to US traders and merchants. Here, with the move against the Spanish Philippines, the initial goal of liberating Cuba expressed in the Teller Amendment seemed to be giving way to a desire for imperialist expansion.