Ralph suggests a signal fire, which the boys light on the top of the mountain. The intention is to add green branches to produce smoke to attract the attention of any ship or plane which they might see.
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The boys light the island on fire to drive Ralph out of the forest. Marines/army people see it and go to the shore where Ralph runs into one. He thinks they are crazy boys playing a game, but Ralph just breaks down and cries, along with the others. Crying for the innocence they lost on the island.
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.
Once Jack is certain that Ralph is hiding in the thicket he puts two different plans into effect. Firstly a boulder is levered from the top of castle rock and sent crashing into the thicket. When this fails to kill Ralph or drive him out of hiding Jack puts his second plan into effect. He lights a fire, hoping to smoke Ralph out of hiding, and unintentionally sets the entire island on fire.
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.
The boys raided Ralph's camp in Ch. 8 because they needed fire. While they were there, they also attempted to do a bit of "recruiting" to further weaken Ralph's clan.