The resistance is the given name of an interstellar military union of multiple refugees of destroyed civilizations that opposed the Harvesters. As of the War of 2016, the Sphere elected the human race to lead the resistance.
According to the Sphere, the Harvesters and their Queens destroyed countless alien civilizations by harvesting their planet's molten core and rendering the planet uninhabitable. The species in which the Sphere was part of managed to hold them off for some time, but was ultimately defeated and leaving the Sphere the sole survivor of its race. Forced to retreat, the Sphere united the remnants of these annihilated civilizations into one strong resistance movement, and leading them in an unending war against the Harvesters.
In July 2016, the Sphere intercepted an Harvester distress signal which revealed their defeat on Earth. Seeing humanity's potential and attempting to save them from the impeding alien retaliation and evacuating them to the resistance's planet, the Sphere traveled to Earth to warn of the attack but was shot down by the humans, who mistook it for an alien threat.
The Sphere was salvaged by the Earth Space Defense just as the Harvesters led by their Queen attacked Earth. Although overwhelmed by the aliens, humanity managed to defeat the invaders by killing the Queen. The Sphere was humbled by humanity's actions and, acknowledging them becoming the first alien-attacked world to successfully kill a Harvester Queen, asked them to lead the Resistance.