Sonic the Hedgehog 2 (8-Bit)
For some time there was peace on South Island. Sonic and Tails spent this time becoming closer friends while Robotnik set about planning how to take down the hedgehog. He was secure knowing that Sonic did not know about the power of the Chaos Emeralds. Between that and the hedgehog's new friendship with Tails, a plan was formed by the mad scientist. In an ambush, the fox was kidnapped by Robotnik and held ransom. The price to free him was that of the six Chaos Emeralds (though Robotnik himself held the sixth given that Tails had it when kidnapped). Seeing as how Sonic would simply see them as gems, it would probably be a price he was willing to pay.
Nonetheless, Robotnik also sent out his Badniks in the hopes of defeating the hedgehog. Without the one person that had defeated him in battle, Robotnik would be free to locate the Chaos Emeralds on his own time. Eventually Sonic managed to collect five Chaos Emeralds and beat back the robot army. His reward? A battle against a robotic doppelganger of Sonic that was powered by the sixth Chaos Emerald. Despite the aesthetic similarities, it ultimately proved to be weaker than the true Sonic and was thus scrapped for the final emerald. Sonic then proceeded to use the six Chaos Emeralds to free Tails from his prison and then took things one step further by trashing Robotnik's Crystal Egg base. He then took off with one Chaos Emeralds and left the others behind (only for the explosive collapse of the Crystal Egg to scatter them across the world).
If anything, this incident brought Sonic and Tails closer together. That and it proved that Robotnik was far from finished with the hedgehog.
Sonic Chaos
Sonic and Tails returned to South Island with the Chaos Emerald found on Cocoa Island. Given that both the Battle Kuku Army and Robotnik were after it, the fox began doing research on the gem that he now knew was part of a group. He came to discover they held a strong amount of power, but was unable to discover anything else of significance before disaster struck again; South Island began to sink.
Attributing this to Robotnik (who also took the chance to move in which his Badnik army), Sonic set out once more to stop him. This time, however, Tails decided to lend a hand with his flight-based abilities. He was also curious to find out just what the human planned to do with the Chaos Emeralds. They tracked down the source of the island's issue to a machine set up in the Electric Egg base hidden on South Island. It didn't take long for the duo to deactivate and destroy the machine. Unsurprisingly, Robotnik escaped following an unsuccessful attempt to kill the hedgehog. Seeing that the fox was also something he would have to deal with now, the scientist realized he would need to think larger with his next plan.
And so he turned his gaze to the stars...