Simplify the following text for a Grade 8 native English-speaking person.
Original: In 1978 , he became assistant conductor at the Kirov Opera , now the Mariinsky Opera , under Yuri Temirkanov , where he made his debut conducting Sergei Prokofiev 's War and Peace .
Simplified: In 1978 , he became assistant conductor at the Kirov Opera , now the Mariinsky Opera . His first conducting appearance was with Sergei Prokofiev 's War and Peace .