Militarily, the Spanish-American War (1898) was not a monumental war. The war was brief, included few battles, and the US generally had an easy time of it, with the war's outcome never in much doubt. Secretary of State John Hay called it a "splendid little war." Internationally, however, the war had major historical significance.
The Spanish-American War signaled the emergence of the US as a great power onto the world stage of international relations and diplomacy. The war did not make the US a great power: the rapid industrialization and economic growth of the previous decades had done that. However, the war did announce to the rest of the world that the US was now a major player. Lifting its head from a century of isolationism and flexing its muscles against the Spanish, the US now transitioned to a vigorous role in world affairs.
The war demonstrated a US move towards imperialism (the taking of colonies). In general, this shift in policy was quite surprising, since the US, once a colony itself, had generally opposed the European colonial habit. Before the Spanish-American War, Congress even passed the Teller Amendment promising that the US would leave Cuba independent. Yet during the war or just after, the US annexed Guam, Hawaii, Puerto Rico, and the Philippines to provide coaling stations for the US Navy throughout the world, as called for by the theories of strategist Alfred T. Mahan. There has been some debate among historians over whether 1898 was a rare moment of US imperialism or the beginning of a long period of informal imperialism accomplished through economic domination. The war also described a pattern extant through much of the 20th century: just as the Philippines and other annexed nations struggled against US rule, US interference in world affairs would not always be welcomed by the smaller nations that fell under Uncle Sam's increasingly tall shadow. The most immediate effect of anger over US interference lay in the 1899-1901 war waged by Emilio Aguinaldo and the Filipinos against the US, which was actually bloodier than the Spanish-American War itself. American embroilment in a military quagmire with an Asian nationalist group over independence seems oddly familiar to later American involvement in Korea and Vietnam, again showing that the Spanish- American War was a sign of things to come.
The war also revealed the growing power of the media to control public opinion in the US. Around the turn of the century and most powerfully just before and during the war, newspapermen like Hearst and Pulitzer practiced yellow journalism, sensationalizing stories and whipping the public into a frenzy for the simple purpose of increasing circulation. There is a great deal of historical proof that the "yellow journalists" tried to instigate the Spanish- American war because they knew war would help sell more newspapers. The role of the newspapers in this war foreshadowed the increasing importance of the media in shaping public opinion regarding wars, as would be seen increasingly in all successive US wars, culminating with the Vietnam War and the Gulf War. In this way, the Spanish-American War was very modern, arguably the first "media war" in American history.
Finally, the Spanish-American War revealed that industrialization in the late 19th century had made the US a great power. Now, as the frontier in the American West disappeared, the nation sought new room to grow: world markets, protected by a worldwide Navy based on island territories throughout the world. Furthermore, new ideas of "Social Darwinism" in the period suggested to many Americans that international relations were a nasty contest in which the "fittest" nations would do what they had to in order to survive. Regardless of the reasons behind annexation, the Spanish-American War and the colonies it brought to the US marked, for good and for ill, the beginning of the modern era of US intervention in world affairs. And the emergence of the US onto the international stage as a power also symbolized to many that the US had finally emerged, whole and healthy, from the era of the Civil War.