Complex sentence: The response was delayed until after a decisive Muslim victory against the Romans in the Levant at the Battle of Yarmuk in 636 Caliph Umar, was then able to transfer forces to the east and resume the offensive against the Sassanids.
Split into simpler sentences: The response was delayed until after a decisive Muslim victory against the Romans in the Levant at the Battle of Yarmuk in 636. Umar was then able to transfer forces to the east and resume the offensive against the Sassanids.