The Resistance-Harvester War is an interstellar conflict between the Resistance, an insterstellar military union of remnants of alien-destroyed civilizations, and the Harvester Empire, currently in the process of being fought. So far, humanity has been the only Resistance member to successfully defeat the aliens twice, hence why the Resistance has offered them the leadership position.
The Harvester Empire is stated to have invaded the Sphere's homeworld, depriving it of its molten core, effectively rendering the planet uninhabitable.
The Sphere's species' remnants retreated to a habitable planet whose location is known only to the Resistance members and humanity (who later became its leader). There, they begam developing superior technology to that of the Harvesters, enclosing the gap between them and depriving the Harvester Empire of its advantage.
Battle of Sol
The Resistance tried to warn and evacuate the humans, using the Sphere as an ambassador. However, humanity thought that the object was hostile and shot it down, despite ESD Director David Levinson efforts to convince the American government that the UFO was not of hostile intent.
The Sphere, however, was salvaged by David and his team, who latter took it to Area 51, where the Sphere revealed the background of the war, also stating that killing a Harvester Queen has never been succeeded before and is impossible.
Despite the Sphere's affirmation, humanity was able to barely, but destructively defeat the Harvester Empire once again by killing the IDR Harvester Queen, which impressed the Sphere so much that it offered leadership position within the Resistance to humanity, which accepted.
Hostilities have currently ceased in the Sol System, but continue in the galaxy. The turn point has been reached with humanity's second victory against its oppressors. The Sphere offered humanity technological knowledge which advanced the human civilization by millennia.