In Chapter 11, why does Jack force Samneric to join his tribe?
Jack forces Samneric to join his tribe to assert his dominance over Ralph. With Piggy gone, Samneric were the only remaining loyal tribe members to Ralph. Jack and Roger pressure Samneric to join the tribe through the threat of violence. The twins fear the repercussions if they do not cooperate with Jack and the hunters. Even when Ralph pleads:
"'Won't you come with me? Three of us--we'd stand a chance.'
After a moment's silence, Sam spoke in a strangled voice.
'You don't know Roger. He's a terror.'
'And the chief--they're both--'
Jack does not want Samneric for any strategic reason or any sort of ability they offer the hunters; rather Jack uses them to send a message to Ralph--that he is now all alone. Jack refuses to allow Ralph any chance of reclaiming his former status as chief. Ultimately, Jack's mandate that Samneric join the tribe is a power play by Jack; it is his way of crushing Ralph and securing his position as leader over all the boys on the island.
In Chapter 11, Ralph, Piggy, and Samneric travel to Castle Rock to retrieve Piggy's glasses. When they stand outside of the fortress, Jack commands his savages to capture Samneric. The savages rush down from Castle Rock, strip the twins of their spears, and tie them up. Golding writes that Jack captured Samneric because he knew Ralph would attempt to rescue them. Ralph and Jack then fight before Piggy is killed by a massive boulder. When Piggy dies, Jack and his savages throw their spears at Ralph and try to kill him. At the end of the chapter, Jack tells Samneric that they must join his tribe. They initially protest, but Roger ominously approaches them in a threatening manner. Jack's reason for forcing the twins to join his tribe demonstrates his tyrannical rule as leader. Jack refuses to have any dissenters on the island and needs each boy to praise him as their leader. Samneric have no choice but to join Jack's tribe and admit that he is their chief.