that was in my opinion the beginning of a phase that concluded with ww1. before 1900 america was already a bigger echonomy than the united kingdom but out of europe's diplomatic and colonial games. after the war the new republican government decided to stay out of european politics (the treaty of versailles was the first important diplomatic meeting where america was represented) but all of europe owed US an enormous amount of money which gave you yanks (assuming you're a yank) also political power and fueled america's industry. you must also consider that with "that little splendid war" (as teddy roosevelt said) america gained a little colonial empire (philippines, guam, cuba, puerto rico) and a zone of influence over central america that made possible the so called "banana wars" for echonomical control over that region. hope that was clear enough, it's a second language for me :)

Keep in mind that at this point in the Spanish Empire, while declining and mismanaged, it was still a beacon of old European power.

It marked the first major defeat between a Western European country and an 'other'. The Russo-Japanese war at the turn of the century would follow in much the same way. Their loss changed the perception of the US both in Europe, and in America itself.

The US gained its first 'colonies', and that illustrated that American expansionism would not stop at the Pacific coast (or at least not the American one). This launched America into an age of imperialism that hasn't really stopped.

To my recollection, the US volunteer militia was so comically mismanaged (with state militias literally racing each other onto ships, and politcal rivals cutting rach other off) that it was the last US war before we Federalized and reformed our USM command and deployment structure. In short, it was the first step in a fifty year process the US adopting a standing army.

It's a widely-agreed tipping-point, coinciding with the USA's "decision for empire" in the ensuing national debate over annexation/occupation of the seized territories. The development of a powerful modern ocean-going battle-fleet started in the 1880s, around the time the US emerged as the world's biggest economy. Germany's challenge to British naval hegemony from 1898/1900 also preoccupied the USA's main competitor, leading to growing US-British co-operation, notably in resisting a European colonial scramble in China. So it was part of a longer process, but 1898 marks the arrival of the US as a world power.

Superpower status would have to wait until WW2, though: here 1945 seems the appropriate date, with US troops on the Elbe and in Tokyo, the USSR preoccupied with securing its regional gains, other rival powers exhausted or largely in ruins, and the US in sole possession of a terrible new weapon. Little of that could have been foreseen in 1939 despite clear US economic leadership.