"Because Homeworld didn't need taking back," Pearl drawls loudly, and even in this state she can still find it in herself somehow to look just slightly irked because this Diya lady is super missing the point. "You're misunderstanding, this wasn't a war for power or resources or territory, it was a war for righteousness and freedom. Homeworld wanted to colonize Earth and turn it into a gem colony, which would have been devastating for the organic life that was already living there." She gestures vaguely at Lapis. "Just ask her. She probably would have been at the forefront of that effort, reshaping the planet's surface for gem habitation. Anyway, that's why we, the Crystal Gems, were of the mind that such flagrant destruction of the native human population was reprehensible. So we rebelled, and we disrupted their efforts, and drove them back to Homeworld to take their colonization efforts elsewhere."

So really it was more like an environmental protest than anything else. A very violent environmental protest.