Complex sentence: The Rumaila Field is an oil field in southern Iraq that also spills over into Kuwait, possession of this field led to disputes between Iraq and Kuwait and was one of reasons for Iraq's invasion of Kuwait in 1990.
Split into simpler sentences: The Rumaila Field is an oil field in southern Iraq, part of which is also in Kuwait. Possession of this field led to disputes between Iraq and Kuwait and was one of reasons for Iraq's invasion of Kuwait in 1990.