Simplify the following text for a Grade 8 native English-speaking person.
Original: What began as a chiefdom in the Samhan confederacies , once allied with China , Silla eventually conquered the other two kingdoms , Baekje in 660 and Goguryeo in 668 .
Simplified: Finally , Silla conquered Baekje and Goguryeo in 660 and 668 each by making a union with Tang , the ancient China country .