how to understand this sentence?

how to understand this sentence?
A disaster is an unforeseen mischance or misadventure(as a shipwreck,a serious railroad accident,or the failure of a great enterprise) which happens either through culpable lack of foresight or through adverse external agency and brings with it destruction ( as of life and property) or ruin (as of projects,careers or great hopes).
怎样理解which happens either through culpable lack of foresight or through adverse external agency and brings with it destruction?