tonight's episode  

[hider=spoilers and theories]So, with this episode, we can get a pretty good idea as to the basics of what happened during the first steps on earth and the first gem war. Like other planets (probably), Homeworld set down on Earth to colonize it and use it to create more Gemkind. This was met with resistance by both the humans and those who sought to protect Earth from the chaos that Homeworld had wrought before to other planets. When Homeworld realized that it was a loosing battle, they sent out a nuke in an attempt to wipe Earth from Gem history, so nobody would ever realize what resistance Homeworld faced on earth, and how they almost lost. This nuke, however, was not to kill. It corrupted all of Gemkind who could feel its effects, Homeworld hoping that the corrupted Gems would be able to take out the humans for them, leaving the earth lost and alone. 

Rose's shield saved her and the crystal gems from the corruption, and the remains of the rebellion did what they could to bubble all of the corrupted gems and keep the earth safe.[/hider]