Honestly I'm not sure if the Night Elves even qualify as a world ranking super power on par with the Alliance, Horde and Nerubian Empire. As both immortals and tree hugging hippies they shouldn't actually have the population to qualify as such. Not to mention a good portion of their power comes from being buddy buddy with Cenarius and his offspring. Honestly they always came off to me as being a power on par with just either just one alliance nation or at best a few of them such as Lordaeron and Quel'thalas combined. Certainly a major power in Kalimdor where their only rival civilizations are either sealed away or nomadic but not a superpower on the world stage.
They're mostly invested in the emerald dream but they(and their allies) were a faction in Warcraft 3.
I figure that they should've been able to build up the required numbers exactly because of their immortality and hippy ways. The Third war cost them a lot of Elven lives and the loss of immortality(which I reckon their population growth got adjusted for culturally) probably wrecked them even harder in the aftermath.
Right now though? It's been almost a millennium since the War of the Shifting Sands and they currently hold more territory than the alliance(and arguably the horde) had at the start of the second war.