Conversation so far:
How did Wonder Girls' Hyuna's departure happen?
Hyuna was withdrawn from Wonder Girls by her parents due to their concern over her problems with chronic gastroenteritis and fainting spells.
What did the Wonder Girls do in 2008?
In February 2008, the Wonder Girls joined their producer Park Jin-young for his month-long concert tour through Korea and the United States as special guests.

Question: What was their breakthrough?
Self-contained rewrite: What was the Wonder Girls' breakthrough?