The Yuuzhan Vong traveled in great worldships to come to the galaxy. When scouts located the galaxy 50 years before the beginning of the invasion, agents had been set up to provide a flow of intelligence to the Yuuzhan Vong. The Praetorite Vong were the first to enter the galaxy, thus beginning the Yuuzhan Vong War. But during the Battle of Helska IV, the yammosk and almost the whole fleet were destroyed, leaving only rogue elements inside the galaxy.