In the year 1900 the two great European nations was Germany and the United Kingdom. Now the United Kingdom had been seen as the greatest of the European powers ever since the Napoleonic wars. France had long ago lost its ability to challenge Britain, Russia could have but had problems with modernization and massive social unrest. The only other European power who could possibly challenge Great Britain was Germany. In 1900 Germany had only been a country for 29 years but it was already a powerhouse. It had the second largest population on the European mainland after Russia, it was the second most industrialized nation in Europe, it had the second largest GNP in Europe after Great Britain, and in terms of industrial output in came third. This is just economics, Germany was also a scientific, cultural, and military powerhouse. And in all of these areas it looked to overtake Britain. French and Russian foreign policy around this time was to try and isolate Germany because they feared its power. Britain was for the time still seen as the greater power, but Germany was not far off.

Now, the United States was also an economic powerhouse. In terms of industrial output they were first, its population dwarfed every nation but Russia, it produced the most iron and steel, first in energy consumption, and they had a huge industrial potential as well as a growing urban population. The reason everyone tended to disregard the United States was because they were isolated and only generally got involved if their sphere of influence in America was tampered with, no one in Europe had ever seen their true potential, they recognized the United States' great industrial strength and potential but since the US was isolationist they overlooked them in favor of the more immediate powerhouse, Germany, who was becoming more and more expansionist and aggressive. The war with Spain helped catapult the USA onto the world stage, but it wasn't until WW1 that the USA was acknowledged as one of the biggest players on the world stage.

Source:

The Struggle for Mastery in Europe by A.J.P Taylor

The Rise and Fall of the Great Powers by Paul Kennedy

Argentina was widely believed to be one of the next major world players, and was well on its way to do so, but decline set in with the loss of trading partners in WWI and subsequent shocks. There is an accessible overview of this in a recent Economist which might lead you to more scholarly literature.