Complex sentence: The invasion of the Soviet Union in June provided new opportunities to use the T4 personnel and many were transferred to the east to begin work on a vastly greater programme of killing: the "final solution of the Jewish question".
Split into simpler sentences: The invasion of the Soviet Union in June provided new opportunities to use the T4 personnel. Many were transferred to the east to begin work on a vastly greater programme of killing: the "final solution of the Jewish question".