Complex sentence: When Boris leaves to pursue a career in the staff of General Mikhail Kutuzov, their friendship evaporates, and at her first ball, Pierre introduces her to Prince Andrei Bolkonsky.
Split into simpler sentences: When Boris leaves to pursue a career in the staff of General Mikhail Kutuzov, their friendship evaporates. At her first ball, Pierre introduces her to Prince Andrei Bolkonsky.