Well in chapter 10 Jack and his tribe of savages invade Ralph's tribe (made of up Ralph, Sam and Eric, Piggy, and one/two littluns) in order to steal Piggy's glasses, which is overall the power to start a fire on the island. Jack might have mentioned this plan in chapter 9 so I'm hoping this is the right answer to your question.
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Jack wants to steal Piggy's specs because they are the only way Jack can make fire
Jack wants to steal something from Ralph and Piggy both. It is Piggy's glasses so he can start a fire with them.
They were fighting with Roger, and Jack stole fire from Piggy and Ralph.
There is no attack by Jack's tribe on Ralph and his followers. Jack led a night-time raiding party, comprising of himself, Roger and Maurice, to steal Piggy's glasses in chapter 10: The Shell and the Glasses.
Jack and his hunters break in and steal piggy's glasses
Ralph opposes Jack's violent ways. He also trying to stop the boys approaching Jack's view of the boys. Moreover, by hunting Ralph, Jack is trying to show who the real leader is and why everyone should follow him.
It's so thickly coated with irony, Ralph's plan throughout the entire book is to keep a fire going to be rescued, Jack could care less. Also, Jack wanted to flush out the island with fire just to finish Ralph off, when instead it brings rescue to Ralph.